"As for etiquette, I'm on the don't-touch-without-permission bandwagon." Abso-bloomin'-lutlely! By just grabbing someone else's instrument the turkey's already shown disrespect for me & my ax.
Be sober and polite and I'm happy to show off my instruments; particularly the acoustic bass guitar that Glen Reid built for me. It's a real treat to hear better players than me (that being most of them) using this instrument.
At a Christmas party this past year, my bass spent most of the evening in Trevor Mills' hands. He even tried to give it back a time or three, but I loved the lines he was laying down, and I think I even learned a thing or two.
And face it, as players, we very seldom hear what our instruments sound like from the front! It can be a considerable treat.