Everyone has bad games, having said that I try to finish the round most times as it's a matter of principle.. Having said that if I am getting spanked from a bad round, down 7 or more shots at the 8th hole of a nine round for example, I simply offer my opponent that I will withdraw if they wish me to so they can move on to another round.
I have never had one yet tell me to go away. The coins don't matter and other things do for some folks. For me I just wish to be polite. I usually on the flip side thank those who don't quit and offer encouragement that then next game will be better. Just the way I see it, YMMV.