I can't think of any pro's for putting wrong-size units into a block. If you're doing your HST's with a trim-down method, you definitely need to trim each one before putting it into a block. If you're not using that method then you need to check each HST to be sure it is the right size and trim if necessary. (If it is too small, figure out what's going wrong and correct it.) Same with the 4-patches. If each unit is the right size before being added to the block you will have far fewer problems later on. With the 4-patches, if they are not coming out the right size, you probably need to be sure the pieces are cut correctly and look at your 1/4 inch seam.