Everytime I show my car in AACA shows (2 or 3 times a year), I will dis-mount my wheels one by one and clean them on back side. I am running the alloy Bundt rims. Also while they are off I can get to the wheel wells for a good cleaning.
BTW, when washing my other two cars ('03 MB 230 SLK, and '08 C300 Sport) I also clean the back of wheels on those cars. Much easer to do so, I don't have to take off wheels because the wheel spokes are wide enough to put a sponge in to get to backside of wheels.
Bob