http://www.openbible.info/topics/tithe_in_the_new_testament shows many portions of the New Testament discuss tithing. I'd almost suggest that at the time tithing was so widely practiced that it simply wasn't a question of doing it - more often the questions surrounded how it was performed, in other words the intention behind one's offerings. For instance doing it grudgingly, or to be seen of others was negative. Also paying a tithe but then turning a blind eye to other commandments/laws that might be seen as more important than tithing.
Certainly that's one valid perspective. Another perspective from, perhaps, the extreme opposite end would be assuming first that all one has and is is due to God's blessings, and in reality He is offering us 100%, and asking us to give 10% back.
Or one can simply accept it as a command given in Malachi 3:8-9,
"Will a man rob God? Yet ye have robbed me. But ye say, Wherein have we robbed thee? In tithes and offerings. Ye are cursed with a curse: for ye have robbed me, even this whole nation."
Still others see it as an opportunity to receive blessings, as the command continues in verses 10 and 11,
"Bring ye all the tithes into the storehouse, that there may be meat in mine house, and prove me now herewith, saith the Lord of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it. And I will rebuke the devourer for your sakes, and he shall not destroy the fruits of your ground; neither shall your vine cast her fruit before the time in the field, saith the Lord of hosts." While the agrarian slant may appeal to farmers more directly, I take it as a promise that my efforts in my career will bear fruit.
I think it's this way with a lot of commandments though - people follow them for a variety of reasons.
Stories abound of people who have had to choose between paying their tithing and paying for some other essential item (mortgage, food, utilities, etc) and having been blessed to manage both after they've chosen tithing. A trial of faith, so to speak. I suppose one might have to test the command to find out if the promised blessings alleviate their financial burdens enough to keep the commandment.
It seems to me to be a question of priority. If you can live off $30k per year, and you get a raise to $35k per year, do you decide to start paying tithing, or do you use that money to fund something else in your life you find important? If you hadn't received that raise, then you'd probably have been able to make $30k continue to work for you. At what point do you have "enough" money, that committing 10% to tithing is reasonable? Some people spend a lot of money on new cars, or cable/satellite service and entertainment that easily exceeds 10% of their budget that a more frugal person might be able to do cheaper, or do without. I'm not saying this is a bad choice in and of itself, but if there are things in your life that are holding you back from a level of religious observance you'd like to have, then it might be worthwhile prioritizing things differently.
I'm making what some might consider a significant amount, but because of the choices we've made as a family things are really tight. We've decided to have a lot of kids and we've decided to purchase a larger house with a chunk of land, for instance. This means that right now because our larger vehicle broke last fall, and since our new baby incurred costs greater than anticipated, we are a one car family, and that car doesn't hold our entire family. We could go into greater debt, or we could stop paying tithing, or we could do a number of other things, but at the moment we are making it work because we don't want to go into debt for a car, and we don't want to stop paying tithing.
I don't think it's unfeasible, but I do think it's difficult, and being a follower of Christ was never described as "easy".
Even if you don't believe in the blessings, and aren't particularly interested in the show of devotion, I suspect the exercise of giving your income to your religious organization forces you to prioritize finances in a way that could benefit you spiritually by removing some of the distractions of life. No netflix? Spend time with your family, meditate, study, or create.
In my church, The Church of Jesus Christ of Latter-Day Saints (LDS, or Mormon), there is no specific direction on this. I pay on gross as I feel that giving the government their due before the Lord His due doesn't fit with my personal beliefs.
When it's come up in discussions with others, one person explained to me that they think of taxes as part of the employer's responsibility, and not as part of their "increase", and indeed in some countries the employers are taxed directly on employee incomes, so the employees don't see taxes taken out of their paycheck (well, not all taxes, anyway). So they pay on net.
I can see value in both arguments, but again in my church while Tithing is important, at the end of the year when you meet with the Bishop the question is, "Are you a full tithe payer?", not "have you paid 10% of your gross/net income", or "how much did you earn this year".
It's up to the individual member to decide what being a full tithe payer is.
In my church it is 10%, but again, individual observance can vary so it's really between the individual and the Lord.
In the LDS church charitable offerings (often called "fast offerings") are completely separate from tithing, and members are expected to contribute 10% to tithing, and then also contribute to fast offerings as able. We encourage members to fast - go without food or drink for 24 hours, two meals - once a month. The money they would have spent on those meals should be given to the fast offering fund. In Malachi, it's not just tithes the Lord is being robbed of, but tithes and offerings.
This fast offerings fund is then used only for local humanitarian purposes, mostly individual families facing food, housing, utility, or medical insecurity.
Worldwide
humanitarian services come out of the tithing fund, and from donations from members directly to the humanitarian aid fund.
At any rate, in the LDS church tithing is separate from charitable donations.
). If you see religion as a personal matter, there's no reason whatsoever to give exactly 10% - for some this would mean going practically without food or necessities, for others it's still a drop in the bucket. It's harder for a minimum wage worker to give $10 than it is for Bill Gates to give away half his fortune (by which I do not want to imply the latter means nothing). 
. In all my religion classes there wasn't any mention about how much should the tithe be. But I seem to remember something in the bible about how a rich man gave lots of money and a poor woman only a few coins. It said that the woman gave more than the man because she was making a sacrifice.
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