Never keep it dirty sorry to hear about your M
+ look at all those dirty cars that go through a car wash. The rollers hold dirt then beat the F out of your car. I took my M5 though a high-end car wash my inside wheels got beat up and my front clear bra was all scratched. I leave my car dirty it drives me crazy or I go to a quick-wash and spray it down.
Many years ago I had a VW and drove it through a gas station wash. The main roller had an old style metal antenna stuck in its weave. When the main roller started beating the car I was trapped by a car behind me, after the wash the car looked like it had hail damage and i was surprised the front glass never broke. After that experience I said I would never use a gas station Car-Wash again.