Why & When for a Scant 1/4 inch Seam?
 

I'm wondering the why of the 1/4 inch seam called for with some pattern instructions. Is it for the width of the thread? Is it to have fabric in the block to square up? Less bulk perhaps?

I try for the true 1/4 inch seam and successful most of the time. I want all of that 1/4 inch of fabric in the seam line to avoid skimpy seams. Most piecing is done using the 2.5 stitch length but sometimes the 2.0 length is used too.

Do you use a scant seam?

to me the purpose is to make the final result fit the size needed. ie the goal is to have a 2.5 square finish to 2 inches square when pieced into the block. I know where to position my needle to get this result when I sew however if I happen to be sewing with a different thread thickness I might have to adjust my seam a bit etc. I don't care whether it is actually scant or by how much as long as it holds together and produces the final result.

A scant seam is used on some blocks because of a particular piecing within the block. A good example is a block called Farmer's Daughter. this is a 12" block but there are pieces across and down in each block so the scant 1/4" is a necessity. The 2 or 3 threads can make all the difference in a perfect block.

The big thing is consistency. I typically sew a bit scant and it can add up by the time a block is more than 6" (and especially by the time a quilt is finished) so I have to trim them periodically.

No one knows but us if our top is an couple of inches bigger or smaller than the directions.

I believe the scant seam is partially a hold over from pre-electric quilting before electric irons and such. At least for me I don't get a flat seam by hand sewing and finger pressing.

Consistency is enough for some quilts, but not when different types of patches need to line up within a block, or when different types of blocks need to line up within a quilt. The important thing, as Pagzz said, is to use a seam allowance that gives the expected measurement for each patch within the block, and therefore for the completed block. Often this means sewing a scant 1/4" from the edge of the fabric because the thread and fabric thickness can mean that a full 1/4" seam allowance will leave less fabric in the patch. It's only a thread or two, not enough to endanger the integrity of the seam.

I think the thickness of the thread also makes a difference. I use Superior's Bottom Line thread in the bobbin and find it makes a difference.

I have found out that as long as I'm using Aurifil thread/50/orange cone, and watch what I'm doing, I will get a good 1/4 seam that is real close to "scant"

I think the bottom line is - as others have said - is the finished size of the patch within the block the expected size?

Factors other than the seam allowance:

The size the pieces were cut - ruler placement, using a die cutter, using a June Tailor thing, did I cut around a template, etc. could make "my" 2.5 inch strip be several threads wider or narrower than "your" 2.5 inch strips/squares.

If fabric was not shrunk before cutting it, did it shrink while it was pressed during the piecing process?

The whole idea of a "scant" 1/4 inch seam annoys me. But I think the pattern makers put that in to discourage the use of "generous" 1/4 inch seams, which could really skew the results of a complex pattern.

When in doubt, doing the three strip test with the thread and fabrics intended is worth the time. Even within the "quilting cotton" category, some fabrics seem to be heftier/bulkier than others.

For me, the fabric preparation (preshrinking by soaking in hot water and then washing it gently) and the cutting are extremely important steps in the piecing process.

I know that when I cut - either around a template or with a ruler, my 2.5 inch strip is actually a couple of threads wider than 2.5 inches.

I do not have a die-cutter, and have not used the June Tailor strip thing, so I do not know how those pieces measure.
Are they "exactly" the measurement or are they a "bit bigger"?

I agree and also, I believe it's much more difficult to be consistent with a 'scant' 1/4" seam.