Joe, are you taking the fenders off? If you are, I would advise using a wax crayon or chalk to mark your line and get some quality shears to trim the fender, try and stay within 1/4" of the line and then use sandpaper to get it to the line. Just be careful not to sand the front because it will uglify the plastic.
You have the sport fenders? (not painted)
Just be careful not to crack the plastic. Shears (or tin snips) would definently work. Even large sicsors.
Also, depending on how you cut them, look into a shaviv deburring tool which will allow you to put a professional looking chamfer on the edge.


Just run it along the edge keeping the pressure even... the tool is around $10.00 and is very useful around the house for deburring pipe (copper, or PVC) and for any holes you might drill into metal.
I use mine all the time. The manufacturers website is
http://www.shaviv.com-Mark
I have three of those with the brass handles; one small, one large and one with a ratchet insert and a
. I have them for work but, I wind up using them for my own stuff quite a bit, too.