I do tithe, but I split it between two churches with very similar doctrines, and between which I've never been able to choose. It's strictly on the honor system and between you and the Creator.
That having been said, the Good Book say one is to tithe "on their increase". So, the way one minister explained it to me, if you take a bus to work daily you can deduct the fare, if you're required to buy tools out of your pocket to perform the job you can deduct the cost of the tools, etc. Anything you have to spend to make your income is not part of your increase. After all of these things are deducted you should tithe 10% on what remains.