She probably does it for expediency's sake. The recommendation that's always made the most sense to me is to be consistent with the tools you use to measure and cut, which means using the same brand of ruler throughout a single project. A mat falls short because it's difficult to cut things in increments of an inch, so I ignore the lines on it. In fact, I often turn my mat over to cut on the non-gridded side because it's less distracting.