How often do you find ticket checkers on buses here?

I've been living in Germany for over a year now between Frankfurt and Munich, and I've used the public transport system extensively. Running into ticket checkers on U, S, RB, RE, ICE, and even trams is pretty normal and common, but I have never seen one on a bus before. Compared to Frankfurt, I always see people here buying bus tickets despite the fact of the lack of checkers, and I'm curious as to what are the chances of being checked on the bus? Have any of you ever been checked before in Munich?

Also, just to clarify, I am not condoning illegally riding the bus without a ticket. I have a semester ticket and have tried to be as diligent as possible in making sure I pay, since I had a few run ins with extremely unsympathetic and cold kontroll people when first arriving here. I've lived most my life in a country and place where the only mode of transport is car, so learning how this whole system works has been an interesting experience to say the least.