The IRS knows to the dollar how much people at your income level typically deduct for these categories. When you claim a higher-than-average amount, it doesn't necessarily mean that you're cheating, but it does raise questions. You could be claiming a much higher medical deduction because your family had a horrible year, healthwise... or that over-the-top contribution could also signify your extreme generosity. It could... but it might also signal that you're not claiming enough income to cover the vast amount you say you're paying in deductible expenses - which can cause problems.
According to a tax research firm, the CCH, in 2007 the average return claimed $26,268 in itemized deductions, which was up 4.5 percent from 2006. For example, returns claiming between $50,000 and $100,000 in income claimed, on average, $7,102 in medical expenses; $6,050 in state and local taxes; $10,659 in interest and $2,693 in charitable deductions, according to the firm, which crunched IRS data to compile these figures.
Being way above average doesn't mean you should leave cash on the table, it just means that you should be extra careful about documenting the deductions and income that you have. If you're significantly under average, you might go back over your credit card statements and checking account records to make sure you're not forgetting items you should be claiming.