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."
We want to make sure that
- Our feedback to contributors is as good as possible
- Our statistics reflect the amount of positive participation we have on the site
- The votes on the site become a guide to the most interesting content
- Our active users gain the privileges needed to manage the site
At the moment, almost all our highest-voted questions are ones from the early days of the site. Of course that was a very active time, but it would be good if some of the interesting questions we've had since then could make it up there too.
I'm not saying we should blindly upvote everything regardless of quality. All I'm suggesting is: If you think a question is well-asked and a good fit for the site, consider upvoting. If you think an answer provides relevant information, consider upvoting. If you find a question or answer interesting or you learn something new, don't forget to upvote!
Of course downvoting has its place too, though sometimes a comment pointing out problems with a post can be more constructive.