I like Aurifil thread for sewing a accurate 1/4 inch seam. It is thinner than most other threads so that when the unit is ironed open, it measures accurately. If you are using a thicker thread, sometimes the unit when opened up, will slightly smaller than required. This is when a scant 1/4 inch seam is needed. The scant allows for the extra fullness in the thread and the folded over fabric.
As long as your units are measuring correctly, you don't need to worry about a scant 1/4 inch seam.