As PatriceJ said, use your rulers not the lines on your cutting mat. My mats are already turned upside down and one of the first things I do when I am teaching/helping someone is turn over their mat too and teach them proper ruler techniques.
I have rather short arms, and I used to have a lot more tummy than I do now. Although I can trim subunits sitting, my cutting is most accurate when I am standing up, my kitchen table is a good height for me. My taller friends often find they do better on the kitchen counter.
I have vision issues and often mark my rulers with tape to help my accuracy and consistency.
I open up every 3rd or so cut to make sure there are no Vs at the fold and that everything is still doing what I want it to do.
I don't starch but I do iron the heck out of my fabric. I mean, can you really expect a lumpy, wrinkly wad of fabric to cut evenly?
And on the other hand, over my last 20 or so years of quilting I've been going more and more to the cut large and trim down style... I find I am more accurate, happier with my blocks and my time spent, and even with the added steps, faster.