Sewing an accurate 1/4" seam allowance is absolutely essential to success in quilt making! If seam allowances are too large, wide, or 'fat'; the finished block will be to small or out of wack (so to speak). Just think about how many pieces and seams there can be in one block.
For example this Clay's Choice Block (above) has 32 seams, in the end the block can become so distorted that it ends up not the intended size or the pieces in the block also will not fit together. If the seam allowance is off by even just a tiny, teeny. weenie bit; these small 1/64" or 1/32" differences when multiplied by at least three, and so many more depending on the block no matter how simple it may seem - well you can see how the inaccuracies can become very troublesome. The same is true of the width of a seam if it is too narrow, only this time the resulting block would be to large.
A scant 1/4" refers to making the width about two threads or 'hairs' less than the very accurate perfect 1/4" seam. This allows for the turn of the fabric when the seam is pressed, either open or to one side. A seam that is a true quilters 1/4" will end up being a scant or a little bit smaller 1/4" usually by two threads or 'hairs. This can make a large difference in how your finished block measures.
(yep it's a pop quiz)
Check your seam allowances!
Do this by sewing three 1 & 1/2" by 3 & 1/2" patches together. Each strip should be cut with the same ruler, this also can make a difference. The finished patch should measure exactly 3 & 1/2" long when finished and pressed.
If your piece is to long, then widen your seam allowance just a bit. If your piece is to short, make a slightly narrower seam. A scant 1/4" seam allowance is ideal when quilting. To make the correct measurement on your sewing machine you can use a piece of painters blue tape on the throat plate or machine bed to use as reference when piecing.
Of course all this being said, quilting is for the fun of it. If you sew a block and its off then don't despair use this as a wink from the artistic spirit in you and create textile art as you so see fit!!!!