Tags in posts seem to be a more prominent feature in PieFed than in Lemmy. Is that more for the federation of the posts and discoverability? Any best practices?

I'm not sure if they federate, but you can go to https://fedinsfw.app/?tag=[tag] and see all posts on the site tagged with that tag or https://fedinsfw.app/c/[community]?tag=[tag] to see tagged posts from a specific community

I think it helps to do a one-click search. I wish you could edit them via moderation for other people though. Once they are set, they are set it seems

I'd say: include the name of the actors, positions, and any interesting features (glasses, dirty talk, stockings, etc).

Yes, they federate and are a good way for people on fedi Software like Mastodon to discover piefed posts (as long as someone on their server follows a piefed community or user).