You should carry a fully loaded pistol. If you get into a shooting the police will want to account for every round that you fired. The easiest and most accurate way to know how many rounds were fired is to count down what's left in your pistol, and then account for shell casings. If the number of remaining rounds plus the number of shell casings equals no more room, it will make your life easier.
This becomes really important if the person you shot (or his friends) starts telling lies (I know they wouldn't), and says that you shot at them on the other side of the parking lot, chased him over to your car, and shot him three more times. Especially if he was in ICU for a couple days before regaining consciousness. This can be reasonable shown to be a lie in one situation, and in the other it leave a lot of room open for doubt.