Of course you should keep a round chambered, for a number of reasons. The first is that under stress, fine motor skills degrade, so the less you have to do and remember to do, the better off you are. Another is the fact that in a gunfight, time is not your friend. Drawing, chambering a round, then acquiring your target and firing takes longer than simply drawing, acquiring your target and firing. I have done drills against proponents of the "Israeli Method" (no round in chamber) and I will have my first pair off before they even get done racking the slide, every time.