Uhhh...Do you mean before punching them? Cuz I really hope you don't mean as you're hitting them!
Seriously tho', open hands while wearing boxing gloves: I have been told, but I don't do it meself, that open hands are more relaxed so they travel faster to the target, where they are made into a fist and go kaboom!
Open hands for MMA: The guy is probably a grappler, and he's going to tie you up or clinch, or he wants to keep you guessing his true intention.
Personally, sometimes I open my hands in boxing to lossen up my forearm, but I keep my fists tight when I get into range, so they're ready to hit .
I think it's probably a bad idea ... signals you are a wrestler and not a striker. Easier to open fists for a takedown than to close them for strikes IMO. Handspeed is more critical to a striker than a wrestler.
Handspeed is faster with open or at least loose hands. You tighten the hands just before impact. Catches and (hand) parries are usually done with open hands...tight fist is a good way to break your fingers (good for knuckle impacts, terrible for palm impacts).
I keep my hands open, I'm not a wrestler and I'm typically not looking to shoot. It does make it easier (for me at least) to parry or catch punches with open hands. I'm more relaxed and I have better movement, also I always look to clinch when I can and if I've always got a fist clinched my forarms are going to be tired quick. It's all personal preference anyway, neither way is wrong.
My hands are usually really loose, sometimes all the way open, sometimes half closed. I move my hands alot when I'm sparring (if I'm ever in a fight I'll probably do the same but I haven't fought yet.), some people say I shouldn't because I leave myself open in certain situations. It's comfortable for me, but when I do get my bell rung I usually keep my hands a little tighter