is it a stupid idea to add a ceiling access panel on the bathroom ceiling above the shower valve /drain area for future plumbing access? Is a plastic one ok or metal ones are better?
Access panels installed in hidden locations like under cabinets or in a closet can be a great idea, but I wouldn't put one in a visible location until its needed. There are repair panels that are made to be easy to install in a drywall hole cut in drywall. You may never need it, so looking a that panel until you do seems extreme.
Plastic would be better in a damp location because metal will almost always rust over time even if it starts with what seems like a very good painted finish. Even if you don't mind the look of the panel the next owner is going to be very suspicious of why the panel is there, and they will not like the look of it.