If you need a visual of why it's better to tear than cut, I have photos on my blog. I tear all wide backs in my shop - due to their width they're more susceptible to being wound 'wonky'. I cut my regular width yardage mainly because more people prefer it that way, although I may tear if I have a bolt that seems really off.
http://andicrafts.wordpress.com/2010...ad-of-cutting/