I have seen some recipes for Terrific Broth that call for the phosphate buffer solution to the filter-sterized then added to the rest of the mixture (which had been autoclaved).  I can't see a reason why one could not separately autoclave the phosphate buffer, as opposed to filter sterilizing it.  Autoclaving won't change phosphate or harm it in any way that I can think of.  Tips?  Thoughts?

That is a good question. Commonly, autoclaving phosphate buffer with other salts can cause problems, such as precipitation (e.g. in presence of iron). I assume that folks had those issues especially with residuals in yeast extract or tryptone. While there are likely batches which should work fine (in fact, you can get TB premixed for autoclaving), the occasional failures have prompted folks to adapt a protocol that is less error-prone. Which makes a lot of sense considering the effort you put in downstream.