Funny enough, I used to tell people the same thing my manager told me... that it was rude to negotiate your first job. Then I talked to @miasanchez about it and she straight up showed me the math on how much I left on the table over 5 years. That conversation changed my whole view on it honestly. Now I tell anyone fresh out of school to at least ask for 10% more, because the worst they can say is no.
Absolutely, that first salary number follows you around forever. I learned the hard way by taking the first offer out of college. You end up playing catch up for years trying to fix it.