You sure you want to cut your rears? That's where all the debris kickup, especially since you're going into MX tracks. Think about the terrain you'll be riding thru... Cut them if not too ruff. Here's some fender cuts.
http://www.bansheehq.com/forums/index.php?...mp;#entry609594
http://www.bansheehq.com/forums/index.php?...ace+cut+fenders