Well as a BMW master tech for almost 7 years now in the Dallas area I think I can answer your questions fairly!
If you are bringing it in for warranty or cust. pay doesn't matter, you definitly do not have to tip the service writer or myself the tech at all. Knowone is expecting it but nobody will turn it down either.
That said...we are there to make money. Your car isn't a 645i or an M3 to droll over to me because I see it all day long. I treat your car as just another job. I still try to treat it as I would my own but no special treatment for sure.
The best way to repay your service writer or your tech is to give them good scores on the CSI survey!!!
Anyting less the the best CSI score you can give your dealer is failing. Sooooo many people think FAIR or OK is a pass which it is not. If you bone the writer on CSI everytime he will avoid writing you up next time.
A good CSI score makes me way more $$$ than your little $20 which some guys love but makes me feel cheap!
If you always hook it up with good CSI and request the same service writer and a good tech to work on your car you will get the special "hey this guy is a good cust so make sure his car is right" treatment everytime. When customers request me by name I take more time, make you aware of things BMW warranty says we shouldn't tell you til it's broke and re-repair things the last rookie butchered.
Otherwise you are getting gravyraped!
What kinda M-car do you drive.
Any idiot can get behind the wheel of a BMW and drive it like they are Hans Stuck. That's why those cars rule. SUV's suck!