For me it really depends on what shop we are talking about. A big box retailer is going to have a ton of people handling a gun which is in their case. They have others in the back which get sold before as the one on display is handled, dry fired, hammer dropped on an empty chamber, slide dropped on an empty chamber etc.... When this is the case i would only buy at a discount.
In a small shop where the gun in the case is the only one in stock I am more inclined to buy the one in the case because as others have said it does not stay there long enough to be abused.
Technically NIB is any gun which has not been shot since it let the factory and has not been transferred to a individual owner.
I have to ask where you releasing the slide using the slide lock on an empty chamber?