Here's my setup. H&R sport springs and 12mm spacers all around on an e90 M3. I went with the same spacers front and rear to keep the handling balance the same, and went with 12mm because my measurements told me I wouldn't get any rubbing in the rear with 12mm spacers, but I would get rubbing with 15mm.
http://www.m3post.com/forums/showthread.php?t=284692