Use mine all the time with no issesmy rulers line up perfect with my mat, do whatever works for you, I ignore the quilt police

I use mine, too. What you want to do is use the same measuring tools all throughout your project. That will ensure consistent sizes.

I use the grid lines to line up the edge of the fabric on and double check the ruler measurements against the grid lines before I cut. I too know the lines of the rulers are the same as the grid lines. My sewing or cutting hasn't been off unless I was using the wrong measurement on the pattern. (Ugh! more than once!)
I line up the straight edge of the fabric. Then I use one ruler on the right side (I am right-handed) to line up my measurement needed along that straight edge of the fabric. (The left edge of the ruler will be onto the fabric.) I make sure the fabric edge is exactly on the measurement of the ruler. Then I take another ruler on the left side and butt it up against the ruler on the right. Double-check the measurements up and down the rulers against the grid lines on the mat for accuracy, slide the ruler on the right out of the way, and cut the fabric on the edge of the ruler that was on the left. Becomes habit to use the ruler markings and to use two rulers for accurate measuring and cutting. When I do it this way I can place the fabric anywhere that is convenient on the mat. Doesn't matter how big of fabric pieces I am cutting, I use 2 rulers for measuring and cutting. For me, the mat grid lines are just for straightening up the fabric, but all the measuring and cutting is done using rulers. I learned to do it this way because lining up the fabric to the 0 marking on the mat drove me nuts. May sound anal but I have to have the measurements on my right, because I am holding down the ruler with my left hand.

When I was in the learning process, one of the first classes I took, my sister in law and I were cutting and using the lines on the mat. The girl who taught the class told us that the lines were not accurate, since that day I do not cut any pieces for a quilt using the lines on the mat, but use my ruler. Now that does not mean that things always line up perfectly, they do not at times. That I blame on my 1/4 inch foot on my machine. I have a time of positioning it just right and a line of tape to get the 1/4 inch. I have found when I start out with something new, as quilting was, if I am shown the correct way I keep on doing it. Just my thoughts. I do use the lines when I want a rough bit of accuracy for some things.

My grandmother cut her quilts using cardboard templates and a tape measure. If it works, it works. Let them do their's the way they want and you do yours the way you want.

I just measured my ruler vs mat tonight...they are the same...use the mat to measure all the time. If it ain't broke...don't fix it! Go with what works for you.

This is the way I was taught. I'm a newish quilter and someone showed me the ruler method which I now mostly use.
I do however 'revert' when I've got a big piece to measure and occasionally for straightening.
Do it the way you like best and are happiest with.

I use the lines on the mat if I am cutting a strip larger than my ruler. The lines on my mat are rather wide and I'm not sure I always line the ruler up in the same spot so I'm concerned about inconsistencies. I prefer to use the lines on the ruler if possible.

it's a personal decision...i use a ruler. i would think that one reason would be that the fold of the fabric is hard to hold down in position to the grid.

Jenny Doan uses her mat as a ruler all the time and she's an awesome quilter. She's featured in 'Quilting Quickly' this month. She has so many videos out and has a thriving online and mortar quilt shop. If she does it, it can't be wrong! She's left handed also.