There's a couple schools of thought on that, and they both have a certain amount of merit worth considering.
The arguments for open:
In the case of over 97% of "defensive gun uses" (the statistic showing how many times a gun is lawfully used in a defensive manner), no shots are even fired and the perp is scared off by the mere presence of an armed person. (Obviously most petty crooks don't have a death wish, and if they think there's a 50/50 chance they might get killed, they're going to bail...not stay and get in a shootout).
So, with that in mind, the idea behind it is that since it's the presence of the risk that deters the bad guy, having one or more people visibly armed in a store would be all the bad guy would need to see to think better of it and say "uh, maybe I'll just go home instead"...essentially, stopping it before it even becomes a situation.
The arguments against it:
In circumstances where it's only one good guy, and multiple assailants, you're at a disadvantage because if they are going to start shooting, you just made it to the top of the target list...they're going to take you out first because they view you as the top threat.
You lose the element of surprise, if an armed gunman rushes the store with his gun already drawn, and sees that you're armed, he's now watching you like a hawk and any move on your part to reach for it is likely going to be risky since the person's now jumpy. As to where, if you're carrying concealed, they're unlikely to pay any special attention to you and focus more on the cashier or their task at hand, which gives you an opportunity to draw when their back is turned.
There are a couple of good videos showing it in practice (I can't post them here due to forum rules because they show people getting shot), but there's a definite tactical advantage to just blending into the crowd until the time is right.
So, with that being said, there's arguments both ways, and there's not one right answer since situations could be different. If you're with a group of 5 or 6 other people and you're all open carrying...sure, that's going to likely stop any situation from occurring in the first place...no mugger or pair of muggers is going to want to take on those odds and they're going to bail...however, if it's just you by yourself, and there's a pair of muggers and they barge into the store with their guns drawn, you openly carrying is likely a detriment because you're the center of attention.