The link to the video below is a great example of why you should always carry your gun with a round chambered. If you have a concern about doing so, make sure you have a quality holster that properly covers the trigger guard to prevent a negligent discharge. LEOs walk around all day with a round chambered in their pistols without any problems.
As the video below shows, if you don’t already have that round chambered, the time it takes to do so may be the difference between life and death. Time is everything in a gunfight. Also, realize that in any given situation, you may not have the ability to use both hands because you have been cut, shot, or your hand may have been broken due to something happening in the incident. You can rack the slide on your gear to chamber a round, but again, that takes time that you do not have.