I'd agree to be careful not to try cutting too many layers at once - I am guilty of this and I tend to over compensate by pushing too hard, which has resulted in many 'non-healing' groves in my mat. I also get so concentrated on pushing that sometimes the ruler slips and I also end up with off strips. All in all, best to take the time to only do a 2 or 3 layers of fabric, making sure you have a new and sharp blade so you're not tempted to push down so hard.