The senior staff in my platoon generally didn't shower with us lowly non-rates and corporals, but after a company run or a PFT, they did. Those events were usually done mid-day, and they needed to clean up before going back to work. All our offices and workspace were in our barracks, so the shower in there was more convenient. Staff sergeants, gunnery sergeants, the master sergeant, and the major used our shower at times like that.

Otherwise, they just showered in their own quarters.