Personally, I think the most important factor is to use the SAME tools/method throughout a project. Consistency is the name of the game. I use the lines on my mat to line up fabric but use the measurements on my ruler for cutting. Unless it is for a cut larger than my ruler then I am forced to use the mat's markings. I don't like the 2-ruler method of measuring - too much slippage for me, plus I don't have 2 of the same manufacturer's rulers so the mat markings work. Those pieces usually are then sub-cut, so I might have a tendency to cut a little on the 'heavy' or larger side (i.e. might end up 12 1/8" vs a true 12") but I can usually use my other rulers to sub-cut to the exact measurements. EXCEPT when doing half square triangles...those I always cut larger (the inch measurement vs the 7/8" measure) and then trim down. Works better for me.