I have no problems at all letting anyone handle/play my instruments. I routinely take them to schools and let the kids "play" with them, (with supervision!!). If a person acts like he is seriously interested, I offer to let them fool with it, but I never ask. I feel that it is up to the owner/player to offer, and that to ask would put them in a tough spot if they don't choose to let others handle their instrument.
I once let my Celtic harp go on an overnite with a woman I had known just 2 hours, and all was fine. She was able to familiarize herself with my string configuration, and give a mini concert the next morning!
No doubt someday I will have to pay a repair bill, but it is truly worth it to see the joy on faces!!