Our guild was making a raffle quilt from blocks that were made by our members, and some of them were oversized and some were up to 1" under sized.
We used a setting that used long triangles on each side of the block to tilt the block a bit. We were able to trim the blocks all to 15" and none of them looked to be a different size. The secret is to use a print fabric that won't show the seams when the blocks are sewn together. I have seen this called twist & turn and topsy turvy.
http://www.maryquilts.com/twisted-happy-block-quilt/
Something like this, but the sashing can be narrower.