For me it depends on WHO is riding and WHICH horse. My bombproof seniors that will tolerate newbie mistakes and not be affected by them, sure - I have lesson kids on them all the time as well as friends, family, whoever wants to just ride a horse. My youngsters, those in training or my personal horse - nope, don't allow anyone on them (aside from my mom on occasion who is a retired trainer) until they are finished and to the point of being un-phased by rider error. I put too much time into them for them to backtrack in training due to another rider's mistake.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.