I found steam to be bad about distorting HST's. For that reason, I quit using steam and just set my iron on the hottest setting...linen. I get a good press and don't have to worry about shrinkage or a block going wonky on me.
Edited to add: The act of just putting the iron on the HST's with steam would actually shrink the fabric and not evenly. I got to practice ripping a lot of blocks and sewing again, with a bigger scant quarter to get the blocks to fit. And, some fabrics shrink more than others.
Second edit: If I have a wrinkle that won't press out, I turn the fabric over and spray a light coating of spray starch on the back side. Then I flip it over and press on the right side. That takes the wrinkle out almost every time. This includes the center fold line.
Last edited by Barb in Louisiana; 08-14-2015 at 02:22 PM.