Having been stabbed seriously in an altercation, I can tell you that sometimes you don't even realize you've been stabbed.... In all the hype of what went down I had no idea I'd been stabbed until my friends were rushing me to the hospital.... that was long AFTER I had completed the mission of kicking the ass of the guy who had broken a bottle over my head (his friend having had stabbed me in the process)... If I had seen a knife come out before, I would probably have given up my mission and moved on FAST, bleeding profusely from a head wound caused by the broken bottle....
So yeah Carl, a knife not being seen might not be effective even after being employed (it wasn't in my case and I was stuck pretty good.... didn't even make me twitch, much less subdue me)... I think you have less to loose by showing it than most might assume,..... unless, of course, you come up against someone who is trained in hand to hand combat... but the odds of that are much slimmer than the odds of the knife being brandished stopping the altercation (I've seen the production of a knife stop a lot of problems without ever being used)....