Is locker room/open shower nudity considered okay?

Is it considered rude to get fully naked (briefly) in a single sex locker room when changing/after showering, or is it considered to be more polite to change under a towel? I know some stereotypes of older guys spend an inappropriate amount of time changing.

As a secondary question, some of the beaches I have been to have showers inside of a men’s change room (not the outdoor pole showers by the sand). The showers don’t have curtains, but are in a single sex change-room. Are these acceptable to use without a bathing suit to get sand off? Or is that crossing a line?