Do income taxes affect my giving?
Yes, they reduce what you actually give to the Lord, and He sees it. In a 30% marginal tax bracket, your tithe of 10% only costs you 7%, thus you deceive yourself and rob God. The great God does not like fudging what you give (Mal 1:8,14). If your intent is to tithe, then you want to make sure it costs you 10%, regardless of how much is received by the object of your giving.
Learn the math. In a 30% marginal tax bracket, if you earn another $10,000, you owe $3000 more in taxes. If you give $1000 to tithe this increase and deduct it on your tax return, you lower your taxes by $300, because your taxable income goes down $1000. Your church gets $1000, but it only cost you $700, because the government reduced your taxes by $300. In this example, you are only giving 7% of your increase, $700 divided by your increase of $10,000, not a tithe.
The vast majority of givers, if not all of them, do not consider the tax consequences of their giving to the Lord, though they certainly want to deduct every penny when it comes time to prepare their tax return. Do the math until you are completely confident of the conclusion above, then determine before God how much as a percentage you will give after deducting tax effects.
More about the IRS, taxation, and giving: God Bless the IRS.