The reason you should not use the grid ruler on most cutting mats is that they tend to be inaccurate due to the thick lines. The longer the cut, the more likely your measurement will be off a bit. I have found this to be true with my Olfa mat and Creative Grids ruler. For example, a 24" cut using my ruler is slightly longer than a 24" cut just using the mat.
HOWEVER - if you watch the Martelli videos, they advise you to do just the opposite - their mats use very fine lines and are 100% accurate. They say let the mat do the work/math! That's why I want one of the Martelli mats - do they ever go on sale?