I misunderstood your original post, so you are trimming the units down and as a result your block is coming out smaller. Got it. In this case, unless your pattern calls for an alternate block to form a secondary pattern, I really don't think it matters. But if there is an alternate block, you will need to adjust it accordingly as well. If you are using sashing you will need to adjust the size of the sashing strips to match your blocks. Ultimately, it doesn't make much difference as long as you are happy with the completed blocks and quilt. Your quilt will just finish slightly smaller than the estimated size using 12 1/2" u/f blocks.