Voting isn't really meant to be about satisfying the rules of the site. We have other mechanisms for discouraging/improving posts that don't follow the rules (like putting on hold).
The intent of votes is that you upvote things which contribute to making this site a valuable resource for you, and downvote things which detract from that goal. In saying this, I'm trusting you to remember that this site is nothing without a community to support it, and a post which you personally do not find useful can still be a good contribution if it helps build the community.
That criterion still leaves a lot of freedom for each individual person to decide how to use their own votes.
For questions, you might want to upvote questions which you find interesting, because presumably if you find them interesting, other people will also find them interesting, so those questions bring more attention to the site - though I would point out that not all attention is necessarily good attention. Or you could upvote questions which seem to be well-researched, under the logic that they'll set a good example for other askers.
You might upvote questions that are at a high level, since they tend to attract professionals. Or you might upvote low-level questions from beginning students of physics, because you want to site to be helpful as a resource to students - though much of the community does not really favor this position, and in fact may prefer to downvote such questions.
If you're like most of us, you'll also want to downvote questions which are "lazy" or don't seem to be well thought out by their askers, because those don't really help the site.
For answers, you'll probably want to upvote answers which clearly and completely answer the questions they were posted on. That's the point of answers, after all. You could upvote answers that show a really good effort on the part of the poster, even if they fall short of answering the question.
It's probably not a good idea to upvote answers that are wrong! Those don't make the site useful at all. You should downvote them. (Though it's not like anyone checks.)
Similarly, it probably makes sense to downvote answers that are correct but completely unclear. Bear in mind that what is unclear to you might be very clear to somebody else, and not every answer has to be useful to everybody. In fact, not every answer has to be useful to the original asker of the question! Think of other people who stop by and see the question later, too.
Because of the community nature of this site, voting (both up and down, as appropriate) helps everybody. I really think people should vote more.
If we have a lot of people casting a lot of votes,
- we can more easily distinguish good content from bad
- we reward good contributors and keep them coming back
- we discourage bad contributors and get them to change their ways (or not come back)
- we also have more people with enough reputation to vote, flag, edit, use review queues, close and reopen questions, and all that other stuff which is necessary to keep the site working!
So you're not polluting the site by upvoting a lot of questions. If you use up your votes, as long as you're using them to distinguish good contributions from bad ones (rather than upvoting everything to make people feel good), that's great! You're helping. We even have a badge for it, to encourage people to do exactly that.