I would never, ever, ever, put a sharp knife on stage. What if the actors hand slipped? With a dull blade, yeah, someone might get a bruise or bump, but they're not going to get cut.
I SMed a show in college where the lead actor had to use a dagger in the final scene to kill someone. The fight was choreoraphed such that he pinned the other actor on his back on the ground, then brought the dagger down and "stabbed" him by stabbing the stage deck basically. Well, one night he decided to "really get into it" (his words) and as he raised the dagger over his head, it flew out of his hand and THROUGH the huge doors muslin door behind him, just a couple feet away from the heads of one of the guards. His parents were in the audience that night; he was trying to play everything up because of them....and got completely carried away. This would ahve seriously injured an actor if it had hit them, but it would have injured them every worse had the blade been sharp.
I would try to find another solution to the other business that has to happen -- you can stab the knife into a table or picture without it being sharp, and perhaps the fabric could just be rigged as breakaway fabric -- so that it's just velcroed (I know, nobody likes the noise, but there is some lo-grade velcro out there that's not so bad) so that it appears as if the actor is cutting it but is really just pulling it apart? Without knowing the show and the business its difficult to offer solutions.
And I think by law, all weapons must remain under locked with limited access only when not in use on stage. Or maybe that's just any kind of gun. I'll have to look into that.