If you have an initial offer from a medium-sized company that includes an above-average salary (in the 50th to 75th percentile, according to the ACS Salary Comparator), how important is it that you negotiate with them for more? The self-help guides all say it's important -- is that right?
Will it damage my reputation if I don't? And if I should negotiate, what should I use as my basis for a higher salary or more benefits when I don't have any data to back it up?First, congratulations -- that's very exciting!
I think the self-help guides are correct, in that they would argue that your leverage will rarely be this high again -- so now is the time to make sure that you get everything you have. It seems to me that there are a number of other things to negotiate for: vacations, 401k/pensions, who will pay for health benefits, etc., etc.
As for damaging your reputation if you don't, it probably depends on who you are negotiating with. If it's a smallish company and you're negotiating with someone you'll be reporting to, it may be good to demonstrate that you're willing to use your leverage. (Certainly, if you take a low-ball offer, they may lose respect for you.) If it's a HR person, they may not care one whit either way.
As for what to use for data as backup, I frankly have no idea. Uh, Google? I dunno. Readers?
UPDATE: To calibrate, this sounds like an entry-level Ph.D. position.