I'm lowered on Eibachs and running stock 19" sport wheels w/Michelins super sports 245/40/19. My M isn't my daily driver and I can now rotate all four (I previously had a staggered set up). However, my fronts are in spec and my rear has a little camber - my decision because I like the traction factor, but not enough camber for me to worry about.
You mentioned that you're on 20's, are the sizes squared all around or staggered? Also, where are you located at? If you're in Southern California, I strongly suggest you contact West End Alignment (in Gardena) - ask for Darin (owner) or Chris (son). They are, by far, the best alignment gurus in the business. They've aligned many of my cars with different suspension set ups and I couldn't be any happier. They have methods of tuning suspensions to fit everyone's need. For example, about a week ago, I put on all new Michelin Super Sports and had the car aligned - they had me sit in the car and make sure the steering wheel was centered and they aligned the car while I was in the car. They aligned the car to my driving needs and style. At the next engine bay, Darin (the owner) was installing coilovers on the new M3 which the owner had staggered 20's...his car was NICE!!! The father and son team are real race enthusiasts and I was fortunate to meet them while I was getting my car dyno'd - we chatted and he offered his services to me since he knew my car was lowered....the rest is history.
in order to get your car aligned properly and in spec you will need:
1. Front adjustable camber arms:
http://www.spcalignment.com/component/s ... m&to=USATo
2. Rear adjustable control arm and bolt (toe kit):
http://www.spcalignment.com/component/s ... m&to=USATo
*optional but recommended: road balanced wheels/tires, front steering tie rods and stabilizer bars, bushings, KYB bump stops
for reference:
http://www.yelp.com/biz/west-end-alignment-gardena
https://www.youtube.com/watch?v=QGC1IlRtoMw