Hi! i know AIM says that pilots are encouraged to visit atc facilities but does not say how or when or which. i went to a tower with my instructor but that was because he had a friend working there. this was like 5 years ago. now i'd like to see another one, or maybe an artcc. shall i just show up at the door with my pilot license and request a tour - or what is the proper approach? i am based in los angeles. thanks!