I'm not sure it makes sense to upvote all the articles you comment on, though maybe it does if you like the post and you're doing a direct comment to it. Comments to comments are as likely to be a discussion between the commenters as directly abut the post.

I recommend downvoting things which are malicious and/or incoherent. I can see a case for downvoting things which add little information (except jokes-- it seems to be part of the culture to upvote things which are funny), especially if they're taking a lot of words to not add information.

I think people acquire a belief that a post or comment of a certain felt quality deserves a rough number of upvotes or downvotes, so they don't add or subtract karma when the post or comment hits that level. I might just be extrapolating from myself on this, but I think posts and comments, especially comments, tend to hit stable karma levels fairly quickly. However, I wouldn't put off giving/deleting karma until you've got that sort of felt sense of typical karma.

If someone mentions in a comment that they've updated a belief, they get some upvoted. This is something I like a lot about LW culture. It's also a handy thing to tell people if you want to explain something likable about rationalism.