There is no special need to do things on your highpoly, if it looks good/looks like it's supposed to then it's good. no need to set smoothing groups on that. on the lowpoly SG question, generally no, read this (wich is at the top of this forum) http://www.polycount.com/forum/showthread.php?t=107196
in the best case the lowpoly would match the highpoly perfectly, so that the surfaces basicly touch each other/are equal, when they are both visible the whole low should be "z-fighting" with the high. But that is obviously not completely possible, it doesn't really matter if sometimes the low is slightly above or slightly…