Thing is Dave, your model may change and then you need more detail on the base, doing the retopo in Z, without having to make a base mesh is a lot more free form and saves a step.

As for UV's I would not even bother unitll the Z sculpt is finished, then do the UV's as they can change if you UV a base mesh then sculpt, again saves a step, and I would prabably do the UV's in Z now with its plug in! Awesome!

Its horses for courses to be fair.