I pretty much take this same stance. I've thought about it a lot - even while in the store waiting to pay.If caught in an armed robbery as you mentioned, assuming for simplicity's sake there is only one bad guy, I would draw my pistol, move to where I had a clear shot, and instruct the perp to drop the weapon. If he complies, great, hold him (assuming a male) at gun point until law enforcement arrives. If he doesn't, and appears to make a move to shoot *me*, I will take the shot.
Here's how I rationalize, if I take the above action and the perp drops the weapon, awesome--just saved some work for LEOs and a big headache for the bank/convenience store, etc. If the perp wants to fight, than hopefully I got behind some cover and can shoot him quicker than he can shoot me--by telling him to put his weapon down with my weapon drawn, I am ahead of the power curve and he is forced to rethink his plan. If I don't draw my firearm and thinks go south, firstly I am at a power disadvantage (no pistol drawn, probably need to move to cover), and secondly people might have died as a result.
Personally, I believe I would sleep more soundly at night (aka, could cope better) if I had killed a perp who wanted to fight and threatened the lives of innocent people, than to sleep knowing I killed a perp and because I waited to draw my pistol, innocent lives were lost. I like to be proactive
I'd be interested to hear the opinions of other OSAers!