The "bow" could be the result of the cutting and/or sewing.
When a the fabric is cut off the bolt, even if the edges on both sides are all lined nicely, its does not mean that it's properly aligned. So if you, at home, don't re-align it and trim it up BEFORE cutting the strips, you're going to get a "v" notch in your strip.
--It's really not the LQS's fault - it's the way it's taken off the rolls and put on bolts for shipping here in the US. And it's much better than it used to be. I remember RJR - especially the Jinny Beyer fabrics - were notorious for this. You could lose over 10% to straightening. They have gotten better, but it does happen.--
Now, when you go to sew in into a strip set, if you don't alternate the direction each time you add a strip to the set, then you could end up with the "J" curve at the bottom. It's got something to do with torque or whatever.
Also, if you sewed a perfect 1/4" seam allowance with the "v" notch, always an even amount from the edge, you're going to end up with a wonky strip set.
And then when you go to cross cut it into the squares, they're not going to be perfect.
My suggestion would be to make sure the SQUARES themselves are perfectly square , and don't worry so much about that each line isn't straight within the block. And there could be some waste because you won't be able to get a good square if the set is really bowed.
With the Rail Fence design, the eye generally travels along the whole pattern and if the different fabrics within a square aren't perfectly equal, this, as we say, will hardly be noticeable from a galloping horse.
Chalk it up to "tuition" and the next time you make a quilt with strips, you'll know to watch out for certain things.
I only learned all that after doing a ton of Bloomin' 9 patch quilts. Scores of strips sets and cross cuts....you sort of figure out why something isn't working, and can correct it.