You've got to realize what the tooth count means. It's not the change in numbers that's important, but the change in proportion. If you want the steps between the gears to be as similar as possible, the difference in tooth count has to change in relation to the number of teeth available.
14/15, a 7% difference
15/17 a 9% difference
17/21 a 8% difference
21/26 a 8% difference
26/32 a 8% difference
Sometimes you'll find cassettes/freewheels with an unproportionally big inner sprocket, sometimes referred to as a bailout gear. It's a trick, or a compromise between having nice and rideable close ratios for most of your riding, and a bigger overall range for the really tough bits.
If you got a triple front, you can often benefit from going to a tighter ratio at the back. Usually the smallest chainring at the front will still provide enough range for most kinds of riding.