I also press each seam as I go. When you lay it on the ironing board, you can usually straighten out a curve right there. I also alternate the stitching direction.
Also, when you do your sub cuts, walk your hand up the ruler so that you are applying pressure opposite the area you are cutting, especially at the top of your cut. The seams hold the ruler up a bit from the fabric allowing it to be pulled by the blade.

Good advice all around. Strip sets are a challenge to keep straight. I usually make them short and sew in opposite directions to keep them straight. It makes for a few more cuts and some material waste but less aggravation.

Even when you alternate the direction when sewing strip sets together it is helpful to use some pins to hold the layers together. Unless you have a duel feed machine or use a walking foot there is a tendency for the bottom layer of fabric to slip a small amount and create the curve which becomes more pronounced with longer and longer strips. A few pins will stop that curvature.

I think she's saying the trouble is a curved strip or dog leg in the strips when cutting - not after sewing. That happens to me when I haven't folded my fabric correctly. And I do have to square up my material after cutting 2-3 strips.

When you sew strips together, you start sewing each strip at the opposite end.........ex - 1st strip sewing starts at top...2nd strip, turn your pieces and start sewing at opposite end............If you start sewing multiple strips all at the same end, your pieces will curve like a letter C..............

I find it helpful to cut my long strips length of fabric instead of width of fabric.

I went through the same frustrating thing with distorted edges. With help from members here, I realized it was the fabric I was using. It was a broadcloth, not 100% cotton fabric. Never again to I use anything but 100% cotton.