I hope this is not a repeat, but: The reason for changing directions that I was given was that most machines feed the bottom fabric slightly ahead of the top fabric when making long seams. The feed dogs grab the bottom fabric more securely. For me, it seems to be true. This can be a big advantage when needing to ease a bit of "extra" fabric. Just place it on the bottom as you sew. It really helps. To avoid the problem, pin often and make sure that the fabric strips start out the same length at cutting. Many new sewers and those in a hurry, ignore the pinning, thereby allowing the slippage to multiply. It is particularly discouraging for a new or young sewer.
Another bowing problem can be created at the pressing stage. Most of us know not to press the iron down hard and push it forward along the seamed line. This heat setting as the fabric stretches can be a cause of lots of bowing, as well. press, lift, press, lift, press, lift, and so on!
I am sure that machine manufacturers would like us to believe that the perfect presser foot pressure eliminates this problem, however, if that were true, none would have started to place the walking feet on for regular sewing!