Originally Posted by Glassquilt
If you cut it lengthwise the fabric will not stretch much.
If you cut it crosswise the fabric will have some stretch.
Suppose you've made a lot of blocks from scraps and the edges all behave differently. lengthwise sashing may give the quilt more stability.
Suppose the same style blocks are just a bit different, crosswise may give you just the fudge factor you need.
I prefer to cut plain fabric on the lengthwise grain (or whichever way stretches less) most of the time.
The crosswise grain (or the one that stretches more) is more forgiving - and one can ease in a slightly oversize block more easily.