If you're using pink insulation foam, you can score through it with an X-acto knife, then snap it apart. Depending on the width of your foam, you can get a perfectly smooth edge that way. It helps a lot if you go very slowly with the knife, sawing with an up and down motion. It works even better if you trace the outline first with a ball-point pen or dull pencil. That's how you can get clean edges on tombstone letters too.
That's what I used. Also, as for carving epitaphs, I used the 45 degree exacto knife method. Just switch blades with each stone, cause they tend to dull quickly! And the electric carving knife it brilliant if you don't have a jig saw!
Well the razor knife route i went. The hardest thing is going in a straight line! I would give anything to make straight lines!
I also think that one of the problems is the blade doesnt exceed the thickness of the foam
I have a hotwire cutter as well, and found I preferred the carving knife. Probably more because I'm familiar with knives more than the hotwire.
Of note...I used to work on an air filter line. The seal is made of foam, and there was flashing that needed to be trimmed. The production line used a scroll saw with a smooth blade. Cut through that stuff like a hot knife through butter.