I suggest you watch Donna from Jordan fabric videos....her technique of preventing the "bowing" is logical and it does work.....assuming one has cut the strips correctly.......

I feel if I cut the strips shorter too that I have less bowing.

Yes, this is what Donna of Jordan fabrics does!

I know Donna from Jordan fabric is a big advocate of finger pressing as you go, and Ironing after in some cases. I wonder if this has any bearing on keeping the strips straight.

I have started using Donna Jordan's method and have much better strip units. I finger press before sewing the next strip on and also do not pull or stretch either strip and allow the machine to pull them through carefully.

I have found myself putting a little extra pressure on strips as I sew them so that they will line up. Even the slightest pressure, plus the grab from the sewing machine feed dogs, will distort the seams. When I let up a little, there is less bowing. I still get some bowing occasionally (sigh!), but a quick shot of steam will usually take care of it.

When do I get the most bowing? When I am in a hurry -- which I usually am -- so I try to slow down a little bit. It usually helps.

Starching helps keep the fabric stable.
Another problem for some is they press the seams by ironing rather than pressing and then lifting the iron and move on. Most irons have a curved side which we use the push the fabric away from the seam and this curve of the iron can cause some distortion if you're not careful. Setting the seam (press the seam from the wrong side before you move the seam allowance to one side helps as well on the pressing issue.
The way your material feeds through the machine (feed dogs, sewer pulling/pushing etc) can also result in some distortion. The shorter the strip the less the issue here.
Cutting can also be the culprit. Things that are not cut on the straight of grain are more likely to distort. It is difficult to get a good true straight of grain on long pieces with cutting. The less volume of fabric you're working with the easier it is to manage and fold it and position your ruler in such a way to try to get it as straight of grain as possible.

I just searched YouTube and can't find the video you are talking about. Would it be possible to post a link?
Thanks,

If you have enough fabric cutting them in half before sewing really helps.

Here is a link to all of her utube tutorials:

https://www.youtube.com/playlist?lis...oGD27nG1AMCCgc