Negotiation is important for a couple of reasons.
First, you want to get everything you are worth. Second, the salary level at which you begin your job will affect your pay increases and every dollar you earn with the company. Never try to negotiate before you have sold yourself to the company, in other words, until you have an offer. Without an offer, you have little, if any bargaining power. You might even find that you've negotiated yourself right out of a job.