|Margaret Islander, in her book. explains how the feed dogs affect ease and I thought I would pass this important info to you all...
"The lower layer of fabric is fed through at a faster rate than the top layer by approximately 1/4 inch every 9 inches although this varies amongst machines, presser foot and fabric type."
Doesn't this explain that perplexing mystery of why one end always ends up shorter than the other?!
"When possible, put the longest , the softest, the slipperiest, the most bias or the most difficult to handle on the bottom so the feed dogs help with job."
A good example of this----
"When sewing a shoulder seam, the back shoulder needs to be eased slightly to the the front shoulder (certainly not on all patterns). by putting the back shoulder agaist the feed dogs, aligning the ends of the seam, and holding as intructed, the feed dogs do the easing for you"
From Islander Sewing Systems book