It really depends on the situation. If all you can negotiate with is HR; talk to them. Learn how to do soft touch negotiation and get the HR person on your side if you can. Just assuming is non-negotiable is a way to lose your freedom before you even start.

Also employees of all stripes would do well to get over thinking that offers are take it or leave it. It is rarely the case. Most people talk themselves out of negotiating or never think its an option.

Hiring is really hard (IMHO) and when you find someone you want to hire you are going to want to make the deal happen.