When I joined Lemmy I used to upvote everything I mild agreed, but then I felt that I couldn't emphasise the truly good comments because I already upvoted everything else.
So now I only upvote the best comments of a thread or a post (or at least of the ones I get to read).
As for down votes, I kinda hate getting down voted, therefore I don't usually down vote just because I disagree, I can disagree and still respect one's opinion (unless it's something reactionary and/or that trully infuriates me).
If I said something that is factually wrong then by all means, down vote so other people may not be mislead by something I thought it was true. But I don't think it' nice to down vote on comments just because you don't share the same opinion on a topic. Or because your personal experience is different from mine.
I also downvote a lot when posts on "technology" communities have nothing to do with (any new) technology and instead are just advertisements for some corporate product.