I think what he's saying goes along with not being a sucker and taking the first offer without first finding out your negotiating position. Hold out a little, or a lot, depending on your counterpart. If nothing else, it's probably good to say "I'm not sure I'm comfortable signing this on such short notice, can I have a few hours to think about it?" and gauge their response. You might immediately make a few extra bucks for just being aware of your options.

Most important is probably knowing the company and the individual you are dealing with. It's easy for people to misconstrue aggression as anger, which may or may not help your case.