I believe what you described was putting cornerstones in the sashing, and it is a great way to keep the blocks lined up. If you do not want to use cornerstones, then do as Jennifer23 suggested. Mark both edges of the long length of sashing EXACTLY where each seam should be. (Here's an example using 8" blocks and 1" sashing--make your first mark at 8 1/4 " and then continue to mark at 1", 8", 1", 8", 1" intervals until your last block which will be marked 8 1/4" from the end of the sashing.)