I never seem to be able to get my HSTs to come out perfect, so I always make them just the bit larger and trim to size. For me, I'm just so much happier with the product it is well worth the time it takes.
Once you have the HSTs, you press open then just line up on the diagonal and trim. The first time trim as little as possible, rotate the block and make sure it is lined up on your 5 inch mark, then trim to size. I put painters tape on my ruler where I need to line up, just makes it a bit easier for me.