I'm a moderator from MathOverflow, and this "question" is actually unsolicited advice, based on our experience from the initial launch of MathOverflow.
We should encourage everyone to vote positively as often as possible!
Every Stack Exchange site will eventually end up with a different "base level" of voting --- that is, the expected number of upvotes for a question of a given level of excellence. (This effect occurs because people see a good question, but already with a certain number of votes, and think "oh, I would have upvoted this, but it already has enough".)
It's easy for us to affect this "base level" by encouraging high levels of upvoting now. We're setting the standards, and this really will have an effect.
(On MathOverflow, we were very active about this early on, specifically encouraging all the initial round of users to vote early and often. You can compare statistics, and see that the average vote total for a MathOverflow question is much higher than on any of the other SE 1.0 sites.)
In case it's not obvious: the rationale for wanting this base level to be high is that it provides better positive feedback to good contributors.