This is the surefire way to keep those seams from moving. Use a needle and thread and tack (sew a stitch or repeat twice in the same place) across the seams where your sewn seam will go through. This way the seam will sew over the tack and you won't have to take it back out.
Anytime I decide to get in a hurry and use a pin I end up having to rip out the seam and start over with a tack. It is a stitch in time saves nine!!
I like to tack the sides as well instead of pin then you don't have to take pins back out. But pinning where there are no seams works too.
It takes a few minutes more to do this instead of pin but I think it is worth it! I lay everything out in rows then turn one row face down on the row beside it and then go through and tack each pair of blocks together. Then I stack the pairs in order and run them through the machine flag style. Saves some time. Then snip and iron.