Is he looking around? Calm? Tweaking? Focused solely on the cashier/money?
Rough plan for that situation is, toss keys to the side, behind him, so he'll look the other way.... If he's armed and the weapon is in play, and not just at his side or something, if I'm close enough, I'm going for a CNS shot. Ie, from 3 feet away, I have enough reach to put a round through his brain. But that's best case scenario, and it never happens how you expect it.
More likely is one or two hoods, who run in, and try to grab the register...
If no one is in imminent harm, then obviously my actions would be more relaxed.. But if someone is in immediate danger, then immediate action is called for.
It's a personal choice... and I don't think I'd be able to handle the idea that I had an opportunity to save someone, but waited, and got someone killed instead.