My understanding is that a 1/4" is a 1/4" and a scant 1/4" is 3/16" - as long as you are consistant in the making of the quilt use whatever you like. If your block has a lot of pieces they say a scant 1/4" helps with mataining the correct block size - you will be doing a lot of ironing your seams and the scant 1/4" helps.