I square up each section of the block as I'm piecing...so that when the sections of the block are put together it *should not* need to be trimmed.
If it doesn't measure up, I figure out where the sewing error occurred in putting the block together and FIX IT! Yes, I take it apart

and restitch

and end up with a square block!
If a block is not kept square throughout the construction of each block, what do you do if there's a 1/4" seam that has to to be there (like in star blocks)? Do you cut off the star tips on star blocks? Do the star tips float away from the seam?
Now if it's a 'free form' type of block (i.e. crumbs, crazy patch), I'll square up at the end of the block.
Nan - IN