The key here is just be consistent! If you cut using the lines on you mat then continue the whole project using the mats lines. If you use the ruler, use the same ruler throughout the whole project to insure better accuracy.

All rulers do not match. And usually they do not match the lines on your mat. My Fiskars ruler lines do not match exactly to the lines on my Omnigrid rulers. Albeit the difference is minute, when working on a project that bit of minute will rear it's ugly head.

All tape measures do not match, either...

Nor do any of these necessarily agree with the little ruler markings on the front of my sewing machine tables. And those of course do not agree with each other!

all lines on mats are not consistent. should not use them for accuracy, only if a smidgen doesn't matter. Some turn the mat over not to confuse the cutting. Most do not suggest using lines on mats. Also if you do notice which side of the line you use for measuring. measure this to see what the measurement is.

This is me too, my ruler and mat are the same brand and the lines match.

I have to use the lines on the mat. The ruler markings are to busy for me and my eyes cross. I have not had any problems doing it that way.

I've heard many times not to use the lines on my mat but I do anyways. Gotta admit though, I always felt a little guilty. :) Why would the lines be there if not to use?

That is so funny! I have used them a couple of times...and I felt really guilty too! :roll:

I didn't think there was any other way to do it...? lol I often cut folded fabric. I line the fold (or straight edge) up on the horizontal line, then match the ruler lines with the mat lines. It seriously never occurred to me that I could cut without using the mat lines...

I always cut by the ruler, not the mat. Most quilters I know do the same, some use only the back side or a plain, no-grid mat. Your mat and rulers may vary so if you use both in your cutting, you will have some differences.
Make a clean cut/start edge and then move to the right to make your cuts, using the ruler only (or the left, if left-handed) Hope this helps! :)

I use the lines to match up with my ruler...but I mostly use the QuiltCut and it uses all mat because there are no lines on the ruler. =)