Well, my original point was that the people I've known who started in academia at the Associate professor level, started at that rank because of their salary demands, not because they were negotiating for a higher rank. I agree that if there is no/little difference in pay, it's better to be an Assistant--mainly because the teaching load may be lighter and the expectations a bit lower. But in some institutions having the rank of Associate does gives definite salary advantages.
I agree that things will be very different across disciplines and institutions -- so much so that fully generalized advice is probably not worth very much. What I can do is tell is how things are at my institution so that the OP and other interested readers can keep these realities in mind and, when pertinent, ascertain whether they apply to them.
At my public R1 (the University of Georgia):
* Tenure and promotion are distinct, formally but also practically in some respects.
* The minimum time in rank for an assistant professor is four years. The minimum time until tenure is five years.
* A raise accompanies promotion but not tenure.
* (In my department) The teaching load increases slightly upon tenure.
* There is no difference in job duties between assistant and associate professors or -- apart from the slight teaching load increase -- between untenured (but tenure track) and tenured faculty. There may be some differences in expectations between untenured and tenured faculty but these seem small compared to differences in motivation and personality between individual faculty members.
* (In my department) I expect that by next year the average salary among assistant professors will be higher than the average salary among associate professors.
* One encounters maximum financial generosity at time of hiring. Indeed, our salaries for assistant professors are slightly above the median whereas our salaries for associate and full professors are well below the median.
* The administration is very resistant to conferring tenure upon hire. (I am told that before about ten years ago it was strictly forbidden and this was enforced at the level of the Board of Regents of the University System of Georgia. It is now possible, but you need to be a real star.)
* In order to hire at the associate professor level, we need to get permission from the administration and put this in the initial job posting. It is not possible (or at least, prohibitively difficult) to be offered an assistant professor job and after negotiations be hired as an associate professor.
If you were already a tenured, associate professor at some other research university making, say, $90K or more, then it would seem implausible to seek employment at my university as an assistant professor. But at my university the sort of job candidate who is on the border between assistant and associate professor levels would gain a financial advantage by being hired as an assistant professor (possibly with a shortened tenure and promotion clock) and then getting the raise that accompanies promotion to associate professor. By my rough calculations doing so would result in approximately $150K more in total salary over the course of a long academic career. This seems to be significant enough to be worth pointing out.
I know that the story I am telling is far from universal. I would be very interested to hear similarly detailed information from those with different experiences, especially with accompanying information about which universities (or types of universities) they are speaking from experience about.