" Standard Slip Differential: Power is applied to both wheels but once one starts slipping, power to the 2nd wheel is limited to the amount sent to the slipping wheel."
This is the main thing that you need to keep in mind. One wheel slipping long before the other can be caused by several things like one being in a mud puddle or off the pavement. Twisting from the terrain can cause one side to have a lot more weight on it than the other, especially in an empty heavy duty truck. In these situations, lock the rear end until you get out of the condition causing the problem then unlock it. You can be more generous with locking the rear end if know that one tire is slipping anyway or if traction on both tires is limited, like when the bed is empty.
I never run with the axle locked without good reason. It is hard on the axle and messes up your steering. It is surprisingly hard to make a turn with the rear axle locked. As others have point out, once both tires start slipping you lose all side to side control so be aware of things that might make you slip sideways.
