KO24, if you use steam and your blocks are on the bias.......they will stretch. Try to use a dry iron and don't move the iron around, just set it over the seam.
If you are a "steamer" then use the steam, but again, DON'T move the iron around. Just set it down, hold it a few seconds, then let the block completely cool to the touch before you try to move it or pick it up.
I've skewed more blocks in using steam than I care to admit. Now I'm a true dry iron hound. I still let the blocks cool before I move them, but I don't get those funny little waves quite as much as I used to.
Happy Quilting,
Sharon