Hi Nigel,
yes, it is absolutely possible to push the errors to 0.0 px error, although not really neccessary. How low depends on your needs. I often get errors of 0.3 or lower without much effort.
The point is (pun intended), that the error points to a difference in what you see and what RC calculates. So moving the CP to where RC says it should be can actually be counterproductive since it might be wrong and you correct. What I do in such cases is that I delete all components and start from scratch. Because if you keep some components RC will go from there and only nudge a bit but it won’t entirely rethink everything.