It is better to "block" the blocks to size than to trim. Sharon Schamber has several good Youtube videos on blocking to size. Here are links to them:
http://www.youtube.com/watch?v=i6aplw_tVZc
http://www.youtube.com/watch?v=sQkF02vpVuw
http://www.youtube.com/watch?v=v-3RIWhBvcA