The only legitimate reason to choose to not vote is if the top candidates are legitimately equally bad in your eyes. This almost never happens.
"I don't like either of them" is not a logical reason not to vote. Choose the one you least dislike. Otherwise you're effectively voting against your interests by denying a vote to someone who would be better for you than the opposition.
(This ignores other reasons not to vote that aren't really a choice. eg. you can't get off work or don't have time or something)