I'm curious to hear other people's thoughts and habits on this!
I upvote if I think the post is contributing to the current conversation, and strong upvote if I think the post will contribute to future and ongoing conversations (IE, its' a comment or post that people should see when browsing the site, aka Stock vs. Flow).
Occasionally, I'll strong upvote/downvote strategically to get a comment more in line with what I think it "deserves", trying to correct a perceived bias of other votes.
I think I upvote mostly like this (I'll edit this answer if I remember more reasons):
Strong upvote: correct (if applicable) and important.
Upvote: correct (if applicable) and slightly important, or not completely correct but interesting/potentially important (In this case I usually also reply). I also tend to upvote comments under my own posts much more because I feel the need to thank the person who took the time to write the comment somehow.
I don't always follow these rules exactly, but mostly. For example, sometimes I upvote to close the gap between two comments I consider equally important under a post.
I vote based on how much I think something contributes to the discussion, aiming for a roughly equal split between upvotes and strong-upvotes (which I expect to be somewhat of a Schelling point). Same logic for downvotes vs strong-downvotes, though I obviously don't split 50/50 between upvotes and downvotes.
I generally upvote if I find myself agreeing with a well-made point or unusually surprised because I haven't thought about it this way before and I find it informative.
I strong-upvote if I feel strongly about my reason to upvote in the first place or if I want to support an opinion/person strategically.
Thus, I see voting pretty pragmatically. Moreover, I don't really think it is that important as my (strong) vote doesn't really carry that much weight at the moment.