+1 with James -- what you buy in the stores is already tempered, and trying to re-temper it is an exercise in futility (plus you risk ruining it if your technique is bad!).
Jerry, to answer your question -- no, there is no value to re-tempering ready-to-eat chocolate.
Back to the OP's original thread question -- if it's really hot out, don't bring chocolate, or bake it into cookies where the melting won't be as nasty, or double/triple bag it and pack it into your sleeping bag.
In any case, as already stated, the higher ratio cocoa solids:cocoa butter bars will be more resistant to melting, and also less sweet (which the OP prefers).
Personally, since I live in the scorching-hot desert, I only bring chocolate in the winter.
Delmar -- not sure if you like shortbread cookies, but I have successfully made dark chocolate-stuffed shortbread. Not too sweet (very buttery), and the cookie outside protects the chocolate inside. Extremely rich, but you're backpacking, right? ;~)