Hendrik notes in the other thread that stretch is "dumb" in that it blindly stretches as you tell it to, I suspect that this will be ok though if it's driven by black bar detection as that detection, if it detected correctly, has the brains and so stretch and just do as it is told.
You should never use stretch, really, unless you very carefully selected your videos and all other settings, as its not designed to retain the proper aspect ratio.
With black bar cropping set properly, even today, preserve aspect ratio should just do the job, safely. Right now black bar cropping is just entirely manual, per video, and limited to a few presets. (well, depending on what you are after, both Preserve and Crop can be valid, depending on if you want constant width or constant height, showing all image content or never showing black bars)
If you throw in anamorphic rendering, you get other concerns of course. I don't assume you just setup a custom resolution with the right aspect ratio, but rather need actually "distorted" output in a standard resolution, so the lens can un-distort it? Which I believe is also possible today, just not very intuitive in the options.
Note that I'm likely to look at how all of these interact as I add support for automatic black bar cropping, and maybe move/duplicate some of it into JRVR profiles. I imagine the existing "aspect ratio override" option sees very little use, because its from a time when screens had non-square pixel, and the descriptions it uses still reflect that. Not many would ever get to the conclusion that it can be used for anamorphic lenses, although thats really quite the same thing as a display with non-square pixel.