I think it really depends on the capabilities of your fronts. If you are happy with their bass quality then perhaps the sub could wait, or you need the rumble then it should come first. Do the fronts project an acceptable center image, if not then perhaps the center is needed first? Lastly, the surrounds may actually be considered on your choice of viewing material. If your viewing preferences involve more surround efx then the surrounds may be needed first. Unfortunately, the decision has to be made specific to the individuals circumstances. I hope I didn't muddy the water too much!