I haven't read all of the replies to this, so someone else may have already said what I'm going to say.
I prefer to add my sashing to each block, instead of long strips, for exactly this reason. So, for example, if I want my sashing to be 2" wide, I sew a strip of sashing material that is 1.5" (includes seam allowances) on ALL sides of each block. Then I sew the blocks together, matching seams, and I end up with 2" of sashing and perfectly matched blocks. This works great, especially when you use a darkish, busy print for sashing.