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 can add up! 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.
Just because you have a 1/4" foot does not mean your sewing a scant 1/4".
Check your seam allowances!
(yep it's a pop quiz)
Cut 3 strips 1 1/2" by 3 1/2", sew these strips together along the long side. The finished block should measure exactly 3 1/2" by 3 1/2" when finished and pressed.
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!!!!