Usually, you will be offered the position over the phone. Hopefully you have had a compensation discussion before this point. I like to think that most recruiters / hiring managers want to give people a fair day’s pay for a fair day’s work. I have offered huge compensation increases to people and I have hired people who actually take a pay cut when they come on board. Sure, some companies may low-ball you but these days I think those companies are more and more rare.
Some people will tell you to always re-negotiate, I am not one of those people. If you have told me throughout the interview process that your number is $45k and I offer you $45k or better than I am expecting you to say yes. If you come back and say your really want $50k it is not a good harbinger of things to come.
Think of it this way… you and your significant other decide to move in. Before you do you two divide up the housework duties and you decide to split the bills. But the day before the move they say, you know, I have been thinking about it. I only want to pay 40% of the rent because I don’t think I will be there a lot. You be like: 
Yep, that is how the employer feels. No bueno. So if you say a number, stick to it. If you negotiate then do so in good faith.
Al
Everything is negotiable. Whether or not the negotiation is easy is another thing.
Carrie Fisher