I'm looking to do the same (add spacers and flares with my 19s for winter) and would be interested also. I found on another chat, someone pointed out that it's best to keep the same line during the winter:

" I want the rear wheels to track through the same line as the fronts when I am driving in the middle of a snow storm. Thats the advantage of a square setup for the winter." - Summer To Winter Pictorial. Spacers? - Bimmerfest - BMW Forums

that makes sense to me...i'm thinking of going 30mm in the rear and 25mm in the front...this way, they track pretty closely in the winter, then in the summer months, i can swap the 25mm from the front to the back and get 10-15mm's (TBD) in the front.

Thoughts?