The important thing is to get the finished unit to be the size you want it to be.

Some of the variables - along with the seam allowance are:

Cut size of the pieces - there can be variations - it is a good idea to use the same ruler or cutting system for the whole project
Was the fabric washed before cutting - some fabrics shrink a bit when they are steam pressed
Fabric thickness
Thread thickness

The moving target explanation is one of the best - and most comprehensive answes - that i've seen.

The three-strip test unit is worth the time if super accuracy is important.

Thanks JanieW for the link to that video. It explains it really well because I have always wondered about the 1/4" vs scant 1/4" too. Now I know.:)

Remember- it is not the size of the seam allowance that is important, but the size of the block!
Sew whatever it takes to get the block correct.

Or, just go with whatever you get and do not change anything and accept what your final dimensions are on your whole piece, quilt, item. If not, then drive yourself nuts chasing the elusive scant 1/4" seam, or obsessing over it.

Consistent seaming, no matter what the seam size, works for many, many quilt patterns. As long as all of the pieces going together are of similar types -- say, all strips and squares, or all half-square triangles -- there is no problem.

The scant 1/4" seam becomes important with more complex blocks -- for example, those that mix squares with triangles, as in some star blocks. At that point if you are not using a scant 1/4" seam, you will start noticing star tips being cut off. The scant seam is also important when sewing complex blocks to each other, to make sure they match up with each other.

It should measure 1 1/2.Because,the scant makes up for the space in the seam the fabrics take up.Another way to try it out,is,cut 2 squares the same size.Put right sides together.Sew E X A C T U A L L Y on the diagonal.Now cut 1/4in. beyond the stitchinq.And press the 1/2 square triangle open.See how your seam is thrown off by the thickness of the fabric.You need to sew just a scant to the right of the diagonal to get it exactly on the diagonal because of the thickness of the fabric.