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.