Good question. Only problem is, there are no hard and fast answers, for two simple issues:
1) Just because you have a firearm in plain sight, it doesn't mean the potential assailant will see it.
2) Even if the potential assailant sees your firearm, it doesn't necessarily mean it will deter them from attacking.
Let's take a look at each of these issues, in turn.
I open carry most of the time, but my firearm is black, my holster's black, and I wear black jeans. Even in well-lighted conditions, such as Walmart, some people simply miss it. I don't OC to "flash my power." For those who do, more power to you, and I know of at least one person here with the personality to carry it off. I would expect to see nothing less than a nickel-plated .45 1911 strapped to his side. As for me, it just doesn't suit my personality.
Back to the first issue: I OC primarily for the ability to draw and fire in the minimum amount of time. I think the recent robbery in Texas at Denny's is a prime example of why speed and accuracy are so important. It wasn't a shooting spree. They were definitely after the money, as they robbed the patrons immediately afterwards, then fled. Given their tactics, it's apparent at least one of them had either military or law-enforcement training. Unfortunately, being aware that some people in Texas carry firearms, they probably agreed to shoot anything that moves. From a tactical standpoint, that's a horrible decision, as they wound up killing one innocent person and wounding another. As they "entered the restaurant and immediately fired several shots," one would have to be very quick and accurate to counter such a threat.
In this circumstance, it's unlikely they would have spotted anyone OC-ing, unless that someone were standing outside. Even then, this was a determined attack, with full knowledge that someone might be shooting back, so this might be the rare instance where OC-ing will get you noticed in a bad way.
On to the second issue: Statistics show that given a chance, a criminal who knows beforehand that a citizen is armed, will tend to avoid the citizen and the business being frequented by that citizen. To what extent is anyone's guess, but stitching together the various reports, it appears to me that if they see it and have enough time to reconsider before making their move, 80%-90% of them will curb their criminal activity. The reports seem to indicate the less time they have to think about it the less likely they are to halt their criminal activity, and roughly half the time, if fired upon, they will fire back.
In the case of Denny's, I think the only deterrent would have been a marked police cruiser parked out front.
Now, let's examine what we know in light of the Trayvon Martin case: Dark, rainy night. Teenager on drugs. I strongly doubt Trayvon would ever have noticed an OC firearm like mine, and a nickel-plated .45 1911 would probably have escaped his notice as well. On the other hand, had Zimmerman been paying attention, he could have drawn as Trayvon walked up to him. Better yet, stay in the car. It's difficult to break into a moving vehicle.
