Unfortunately I forgot who said it, but box cutters are for just that, cutting open a box. I exclusively use a round knife and on very few occasions a rotary cutter. You have to keep the round knife sharp at all times; and some people don't like to do that.
I have a metal ruler and a metal square I use when cutting straight lines.
Below are a few links to Tandy. I've heard there are better, but you can use these links as a starting point. You can pick up a Rotary Cutter at a local Walmart for a lot cheaper.
Round Knife
Rotary Cutter
Hope it helps.
Butch