When I do on point blocks with setting triangles I startch heavely and after each block is sewn I have a piece of heavy duty broadcloth with the finished block size drawn on it which I put on my irong board. I lay the block on this "pattern" back side up and if it matches the size, great, if not I dampen the block with water and use straight pins pushed straight down or angled into the iorning board padding to hold the block in place as I coax the block into shape and then press it.
I also sew each block seam and row seam in opposite directions. this allows for the movement in the pieced blocks when sewiing. If the blocks and rows are all sewn in the same direction they have a tendency to be pulled in that direction. Hope this gives some help.