Negotiating should be attempted, unless they tell you up-front it is not negotiable. In that case, you take it or leave it.
Always consider how much you need (or want) that job, vs. how much they need it filled and how much they value you above other applicants. I have hired many people at this point; I am always willing to pay more for someone I really want, but not willing to up the offer substantially for someone I think I am settling for. A few thousand bucks either way is always worth it just to close a deal and move forward. At the end of the day, everyone has to come away feeling good about it, and that they came away winners. If there is too much leverage in either direction, one party will really get what they want, but it is likely to be temporary as the other party will be looking for ways to improve their position ASAP.
Also consider other aspects to the negotiation:
Will they pay moving expenses?
Will they pay a sign-on bonus?
Will they cover insurance (health/disability/life)?
Will there be annual bonuses? what about the rate? How is it structured (your performance vs. company performance)?
If industry or large lab, is there equity? how much? What is it worth?
Good luck