You
shouldn't vote if:
All candidates suck. (See also: http://unquietmind.com/voter.html)
You don't have the time/interest to learn about the issues and candidates.
You're planning on selecting a candidate to vote for based on one or two issues.
You're under-aged or not a citizen.
You don't agree with me on every issue. (Eric Harris said it best.)
(IOW: If you don't have anything to say, don't say anything.)
BTW, even if you don't vote, you have every right to complain. But we can complain about you not voting too. :-)