When I use the June Taylor slotted ruler, I usually sew a 'SCANT' 1/4" seam.
If you cut a 2" strip with your regular ruler and then one with JT, you might find that they are off just a little. IMHO . . . ask me how I came to this conclusion. I do tend to cut my strips (reg ruler) probably on the thicker side of 2".
Also (more thinking out loud) sometime we have to make adjustments on our 1/4" seams because of the fabrics. Have you noticed that some 'seem' to be thicker then others.
Between the rulers, the fabric, and our sewing, and machines, it is always best to check the 1/4" seams.
I could be wrong, but some food for thought.