I guess you and I will disagree. I definitely include my tithing and church contributions as a tax deduction. And I think 99%+ of other members do the same thing. But if that's how you feel - that's completely up to you. The Church's position is that it qualifies as a tax deduction given all the effort the Church goes through to provide documentation proving that it is a tax deduction. But that's my opinion.

The Church of Jesus Christ of Latter-day Saints is a non profit organization. So all contributions are tax deductible, meaning that under the law that sacrifice reduces your taxable income.  
 
By not claiming the tax deduction all you are doing is artificially inflating your legal income. Properly reporting to the IRS your charitable contributions is not a sin nor a crime. The real crime is to be over paying on your takes because you are falsely reporting additional income that you do not have.

You should deduct your tithing from your income taxes unless your income justifies taking the standard deduction instead. Congress knew when they made the tax code that there were religious groups such as ours where the members paid 10% of their income to that group. They also knew that these groups helped to lift heavy burdens off the shoulders of the Federal Government. Take our church for example, a lot less of our church membership uses government assistance programs because the church has programs that are better made possible by your contributions. You should also deduct any fast offerings and even donations made to the Boy Scouts because they qualify too.

Tithing as I understand it should be on your gross income and claiming it on your tax return is just fine. It is a legal bonus we are given, no crime in taking it.

This is a question more about how you view the government than anything else. Under the Internal Revenue Code there are taxes imposed on certain conduct and exclusions from taxable income for certain conduct. As it turns out one particular type of conduct -- donating to a charitable entity -- is a basis for a tax deduction. I think the question has a logical fallacy in it -- that we should not get a material benefit for fulfilling our religious covenants. Perhaps we should not do them in order to get the material benefit, and it is probably a bad sign if we would not do them but for the material benefit. However, I can't think of a good rationale for not taking the deduction just because you made the tithing donations without being motivated by the material benefits. In essence you are rendering unto God what is God's and Ceaser is saying that if you do that what is Ceasers is a little less. That said, there is no requirement that you take the maximum tax deduction to which you are entitled. If you want to effectively donate money to the United States, which is a very noble and patriotic and, incidentally, exceedingly rare thing to do, you can effectively do that by not claiming the deduction for charitable contributions based on your tithing. Whether you take it and then donate to the United States Government -- which is also a charitable organization for which you can get a deduction -- or just don't claim the deduction the effect is the same. If you want to donate to the government you can go on pay.gov and they will send you a receipt for your tax deductible gift. 
 
As to the guest who says that tithing should be on your gross income, the general authorities have repeatedly declined to give any definition of what tithing is beyond to say it is on your increase. There is nothing magical about gross income that makes it the right number. If you mow your lawn you are increased by the amount you would have to pay someone else to mow it? I think you are in the same way that early church members were increased to the extent of the food they grew. Do you pay tithing every time you mow your lawn? I'm not saying you should, just saying there are all sorts of ways to define increase and gross income is just one of the many ways to define it, as is net income on your paycheck or whatever else you believe is correct between you and the Lord. 
Before most increase was specialized and monetized like it is in our economy the question of increase had no artificial basis such as tax documents and I think there is some simplistic beauty in asking yourself how much you have been increased and paying on that rather than on what your pay stub says your gross income is. IN any event I think we should all respect the fact that the general authorities do not select among those many definitions and have left that to everyone. We ought to resist preaching our own personal interpretation (gross income) as though it were church doctrine or the actual covenant we have entered.