I know several good guitar players who will allow 'people they know & trust' to play their guitars....and have known one or two who will seldom or never. Sometimes it just boils down to the occasion and surroundings....and the particular guitar. I DO know that any guitar player who picks up another's instrument without asking is likely to be crossed off the list!

(I have it easy... I play autoharp, and it's hard to hurt one.)