Is there anything wrong with upvoting all of the answers to a single question? I realized that I've done this several times already, because I liked all of the answers. On the one hand, אלו ואלו, כפטיש יפוצץ סלע etc. there can be value to all of the answers given. On the other hand, since part of the reason for upvoting is to differentiate between the usefulness of different answers, maybe upvoting them all is inappropriate.
I try to evaluate an answer against the question, not against other answers. If this were the only answer would I vote on it? If so, I should vote -- why is it the author's fault that, between his posting and my seeing it, someone else added another good answer?1
However, I don't ignore other answers entirely -- if a new answer covers the same ground as already-existing answers, then even if it's a good answer I don't necessarily upvote it because it didn't bring anything new. There's a lot of hand-waving judgement in this.
1 Sometimes this means I've upvoted (or, more rarely, downvoted) most of the answers on a question. It happens. If it happens a lot, it might be time to ask yourself whether you're happy with your voting criteria in general, but that's a private matter. (I've done that -- observed patterns in my own voting that make me stop and think, and changed voting behavior as a result.)