First, I'm going to be stating an opinion, so let me tell you all my qualifications for my opinion. I've had classes in the defensive use of handguns from Bennie Cooley (
http://www.benniecooley.com/), Gunsite (
http://www.gunsite.com/ ), the Walt Marshall people (
http://www.awt-co.com/), Louis Awerbuck (
http://www.yfainc.com/ ), and Massad Ayoob (
http://www.ayoob.com/ ). All of these trainers/schools teach one to carry his sidearm with a round in the chamber. I've also competed in USPSA. And I'm an NRA Certified Instructor in Basic Handgun, Personal Protection Inside the Home, Personal Protection Outside the Home, Home Firearm Safety and Shotgun.
Okay, here's the bottom line. If you carry without a round in the chamber, you will either need two hands to make the gun ready for use or need to figure out how to chamber a round with only one hand. The latter can be done, but it is difficult and slow.
It is, in fact, extremely unlikely that you will ever need to use your gun in self defense. Most people will live their entire lives without ever having a violent encounter. But, based on my training, it's my belief that if you do need your gun, there is a strong possibility that you will need it quickly. Criminals like the element of surprise.
In any case, you can not predict how your particular emergency, if you ever have one, will arise. Maybe you'll have some time to make your pistol ready, and maybe you won't. Maybe you'll have both hands available, and maybe you won't. In addition, if you need your gun and need it quickly, the consequences to you of not being able to make it ready quickly could be horrible. In other words, there may be a small chance of an emergency, but if you can not respond appropriately and timely, your outcome will almost certainly be very, very bad for you.
Of course, if you are going to carry a gun at all, it's important to get good training and practice well. Training shows you what to practice. Learn to handle your gun properly and safety. Keep your finger off the trigger, and indexes on the frame, until you're going to shoot. Use a good holster that covers the trigger guard. Get good training.
And did I mention that you should get good training.
A violent encounter is a very high stress situation. Good training and diligent practice will help prepare you to respond effectively in a high stress situation. In a high stress situation, one defaults to his level of training.
As for me, I will not count on having both hands available in an emergency, so I carry my gun with a round in the chamber.