Pay at the end, 100% of the time under normal circumstances. I am , however, quite willing to display the cash, put it on the dresser clearly visible, etc etc, so that she know it's there and for her. It does not go into her hand until the end of the session, period. If she insists then I walk. I do admit to being more of a product of international hobbying though, where it's a hard and fast rule that you never, ever, under any circumstances, pay up front and anyone who does is an idiot. I would very likely be more flexible with the ladies of MERB. It absolutely boggles my mind that anyone would prepay tips though. I think that's just insane. I can't even imagine prepaying waiter or taxi tips prior to receiving service and I can't see why this would be any different.