I've used a regular box cutter with the blade you flip once and then dispose of once both sides have become dull. I prefer several dedicated leather knives, a Japanese cutting knife, for one, and a brass leather pen knife, for another.
I've decided to give the box cutter with snap-off blades a try. What snap-off length do you use?
I have Grants and a cutter made in Japan. There is blade runout.
I use a small Stanley box cuter with Japanese disposable blades