“i consider it one thing to pick up a guitar. for one thing, i know that that's okay in this shop. for another thing, playing unplugged (especially if it's an electric) doesn't create enough noise to bother anyone else in the store. OTOH, every person that plugs in of his own volition increases the noise level in the shop. my approach is to let them decide how much noise, how many people are playing at any one time.
I've also noticed that no matter what I'm looking at the salesman just happens to have the exact same thing at home and loves it.