Spiros recently touched on this, as did Philospohy Smoker, with a cautionary example (short version:  candidate asked SLAC for a bunch of things, and the SLAC simply withdrew the offer).  My own view is that if you have other offers, you can negotiate.  Otherwise, all you can really do is get clear about policies, and ask about one or two things that might be especially important, in the spirit of "this would very nice, is it possible?"   But "negotiation" and "bargaining" is only really a posture to adopt if one has another option in hand and one is prepared to walk away from the offer.  Or so it seems to me.  (If there's enough interest, I'll open a thread for comments on this next week when I have more regular computer access.)