I hardly ever say "no" to lending my fiddle, and then only when the person is extremely drunk and uncoordinated, in "show off" mode, or simply if I want to play it at that moment. Other than the drunks and show-offs, people tend to be careful. Musicians just know they should "handle gently," and non-musicians often treat the fiddle with a certain reverence. If someone who was trying to be careful had an honest-to-goodness accident, I doubt I'd get mad at the person or quit lending instruments. I have only one violin I consider to be irreplacable, but although it's precious to me, it isn't sacred. While in my care, it's been sat on, spilled on, scratched, dinged, thumped, and left in a bar. Maybe the thing is actually safer with someone else!