Consider a 3 way race:
Pat Robertson vs Anita Bryant vs Mohammad Omar. Who would your choice be?
I could not vote for any of them for any office. I suspect you're the same way.
"If you choose not to decide, you still have made a choice" - "Freewill" - Neil Peart. :-) Sometimes, not voting for the major candidate, or writing in someone you know has no chance, is the best choice.
Voting for Nader as we did in the last presidential election was effectively the same as not voting. It had no impact on who won in my state of Virginia, and I don't think it affected the outcome in Washington state either. It might have, accidently, affect the outcome in Floridia (due to mistakes some people made in filling out their ballots) - but that assumes people who voted for Nader would have voted for Gore rather than staying home (something I don't think is a good assumption). I suspect that if you believed that it was important for your vote to always have a positive impact on who won, you would always vote for one of the top two major candidates. But you didn't, so you don't. Right?
My $0.02.
Cheers,
Scott.