You may be using a guillotine, I don't know, never having used one.
I employ a sliding "trimmer" (Fiskar)
downside, good ones not cheap Circa $30 (albeit cheap trimmers do exist)
I can cut stamps in the mounts (cut thousands without a mistake)
You can trim say a 2mm strip, if required, they are that accurate.
Cutting stamps in a strip, or singularly with the base butted to the left edge,
gives you a sharp 90 degree angle every time.
http://www2.fiskars.com/Products/Cr...r-Trimmer-12