People, it doesn't have anything to do with the patterns. All it has to do with is your own machine settings, the thread you use, the way you press and the way you cut your pieces.
The point is to have the finished piece, be it block or quilt, come out to the planned size. There are many, many ways to adjust any of the factors that affect block size and if you want to end up with a perfect 12½" block every time, you'll do what you need to for that to happen. If you don't care and prefer to just sew consistent seams, that's fine, too. It's totally your call.
The only reason patterns says "scant ¼" is because people were complaining about their blocks coming out too small for all the reason mentioned elsewhere. The pattern wording has nothing to do with it and there's no way the writer can make you sew perfect measurements. If your finished blocks are too small, they'll be too small whether the pattern said to use a scant ¼" or not. Would you prefer they say to cut the pieces a "tad" larger than the measurements listed?
Do a test block, measure it and make whatever adjustments you are comfortable with to make sure the size is what you want it to be. It's an easy habit to get into...and can save frustration and fabric galore! It's exactly like testing the gauge of knitting needles and yarn before you start a new sweater.
There is NO finite measure for a "scant" quarter inch, just as there is no finite measure for a "dash" of salt or a "dollop" of whipped cream.