To my knowledge bags on the bottom is the rule of thumb...feed dogs eat it....

The term I learned (when sewing with knits) was to "sew with a relaxed bottom" - meaning that the "longer" piece/layer was next to the feed dogs and the top layer was "stretched" to fit the longer piece below. This usually works with most woven fabrics, too.

This only works up to a point before it looks like gathering instead of easing.