This isn't clear to me. I haven't been using them very well (similar posts end up getting different tags because I'm lazy) but should I go nuts with every conceivable tag, or keep it short? I've been using 10-15 tags per post.
They won't hurt, so just go for it. If you show all of your tags on your theme, then just make sure it doesn't look the theme look bad. Other than that using many tags shouldn't be a problem.