Although this is about LaTeX (not AMS-LaTeX) and is from 2014 (and I don't remember when this AMS-LaTeXism was introduced), nowadays instead of having to decide when to use `\ldots` and when to use `\cdots` one can use things like `\dotsb` for dots between binary operators and `\dotsc` for dots between commas. There are others that I don't remember, because those fulfill most of my needs. There's even `\dots`, which tries to determine what kind of dots are appropriate from context (and usually does a good job!).