I had always thought that I had a good 1/4 inch seam. I made an illusion quilt that was a large circle made up of 48 wedges. In the center the tip of all of the wedges come together. The class instructor stressed testing you 1/4 inch seam. I like everyone else said "I have a 1/4 inch seam." well guess what I didn't I was 2 clicks of the needle off. The instructor showed us on her class sample the importance of this. Her circle was pretty puckered and would not lay flat. No way she could quilt it.
I checked my seam and checked it often as I made the circle. My circle went together with only a slight pucker in the middle which was easy to fix.
Being off a tiny bit doesn't hurt much on some blocks and you can fudge it to make it fit, but on other blocks it is a huge difference. My quilt guild just made a donation quilt. We passed out 40 block kits ( pre-cut). It was supposed to finish at 12 1/2 inches. Blocks varried from 13 inches down to 10 1/4 inches. We actually did a demo on 1/4 inch seams and passed out how to check it the night we passed out the blocks.