Um - okay, I had taken the time to type out a reply but apparently the editor decided to have it lunch!
Leonard Zinn, authro of Zinn & the Art of Road Bike Maintenance, posted an article on Velonews.com at some point on how to cut carbon steerers. He suggests that even if you have a Park SG-6 Threadless Fork Saw Guide (http://www.parktool.com/tools/SG_6.shtml), you should make two cuts - one from each side - in order to avoid fraying/fracturing the carbon.
Michael
¯\_(ツ)_/¯