When worldising dialogue, would you leave that to the mixer or the editor or sfx?
Depends on what you're trying to achieve…
If you're talking about "worldizing" in the traditional sense, ie. taking a pre-recorded sound and re-recording it in an acoustic space, then the mixer is not really the right candidate since they deal mostly with console reverbs, Altiverb, etc. If it were me, I'd do it myself. That way, you're learning how to do something new, you're gaining experience, you're leaving room for trial and error, and you're adding new material to your library.
Not to mention, you know exactly what you want it to sound like, so why waste time trying to explain it to somebody else?
I would leave worldizing to a SFX guy. I am one and I want all dialogue worldized because too many times have I see the hero in the woods talking like he is in some studio somewhere.
I even worldize audiobooks productions, simply because it drives me crazy to hear studio dry voices. Only time it is okay is at narration.
So to answer the question, leave it to the SFX guy, we tend to know how reverb works and such. Not saying that others don't, of course.