I have blocked blocks on several occasions as GingerK describes, only I do mine on a cork board that has a true 12 1/2" square marked on it. I line the out of shape block up on the square using loads of pins to stretch or shrink it then thoroughly wet it with a spray mist bottle and leave it to dry overnight. It comes in very handy with block swap blocks where not every block is a uniform 12 1/2". There is absolutely nothing wrong with doing this. However, I limit it to blocks that are off less than 1/4" max plus or minus. Anything over that I won't use the block. I only do this to completed blocks, not units to get the block to fit together. Blocking a block and then a whole quilt when it is done is a very common practice.
Additionally I have known many longarm quilters who use starch and or steam while it is on the rack to help draw up fullness in a wonky quilt so they can quilt it without getting unsightly tucks or puckers. I have never done it myself but I have definitely had quilts on my rack that could have benefited from it.
Edited to add, I can attest that the quilting will stabilize a block this has been done to. Actually, in my experience, once the block has been blocked and dried, once I unpin it it keeps the shape so the quilting only enforces that.
Last edited by feline fanatic; 01-29-2016 at 12:23 PM.