To the dark side. It keeps the batting from showing thru. Also -- if someone does a stitch in the ditch, then you need it to one side. I do longarm quilting, having the seams to one side does not bother me at all -- the "bulk" isn't a problem unless you've got 6 points at one corner. And even then, I use a hammer and pound the fabric down. Very seldom does it cause my machine to jerk out of line.