Time it takes: 20 minutesKnowing what rate you pay on the last dollar you earn can help you to, among other things, pick a taxable vs. a municipal bond fund. Pull out your most recent 1040 and look for taxable income (line 43 in 2006). Adjust for any big changes in your income or the deductions you expect to take this year, then find where you fit in at irs.gov (search for "2007 federal tax rate schedule").
To choose between a muni and a taxable fund, divide the muni's yield by 100% minus your tax rate. If that number is higher than the taxable yield, go tax-free.