I just dealt with this same thing. If you are pressing the seams to one side or the other, the amount of space it takes up to roll one of the sides up and over the seam allowance is what the scant 1/4" seam is for. (I'm filling in with some hand gestures to punctuate my explanation, but I doubt you are able to see that! :D ) It is approximately the amount of the width of a couple of threads on the fabric. On my machine, with my 1/4" foot, I sew the seam in such a way that I cannot see the edge of the fabric, but just the edge of the feed dogs. You have to find the "sweet spot" on your own machine, as it seems each machine is different. Sew a block and measure it to see how accurate your seams are, then make adjustments accordingly.
M