In general, I tend to be or the 'more tags is better' school. I only use them at AO3, since I pretty much only post there and my writing journal (mostly with AO3 links ;p). At A03, Tags are really meant to help users find things in searches, so it is useful to think about what kinds of things readers might be looking for - porn and kink tags will draw readers into almost any fandom, I find. There are tags for pretty much anything you might want to warn for in a story beyond the general archive warnings. Tag for anything you think might draw someone into a story, whether it be the setting, the relationships between people (like 'Married couple' or 'established relationship' kinds of relationships). And do add tags that don't exist yet! Wranglers will take care of them :)