There are always exceptions. Photoman pointed out one. If you can't carry in a holster for some reason, and there is more possibility that the gun would go off by accident with its five pound trigger than that you would have to use it, your odds are probably in favor of not carrying with a round in the chamber. That is a special case, and I bet Photoman more often than not does carry with one in the chamber.

Violent incidents can happen so quickly that you would be lucky to even get your gun out of your holster before you are killed in a lot of circumstances. Your opponent isn't going to stand still while you take a second or two to rack your slide, if you still even have two usable hands to rack it with. That is standard procedure. But, as Photoman pointed out, and his was a good example, sometimes you don't get to do what you'd like to do and decide that it is better to have a gun with the round not chambered than to not have the gun.

If you are not comfortable carrying with a round chambered under ideal circumstances, meaning normal everyday carry in a good holster, you need to get comfortable with it. Start carrying around the house with a chambered round until you can wrap your head around the concept. All of this supposes you have a normal carry gun that is in common use for self defense. If you are carrying a Single Action Army, for instance, carrying with a round under the hammer is not a good idea at all!

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Peter
S&WCA #2620