It depends, but the norm is about 15 degrees of angle to maximize incoming power. See, the panels only produce maximum power when facing exactly at the sun, and since the sun is never exactly directly above you (unless it is high-noon and you are on the equator) you need a little angle.
Is it worth it? That is dependent on your setup. If you are not getting fully charged each day with your current setup, a tilt can get you another 20% (or more) incoming power over the day. Myself, I have enough panels on the top of my van that I am usually charged up by noon anyway, so a tilt is unnecessary.