OK, thanks all for the comments.  I think for now, at least, I'll stick with 720x480 since, as I mentioned, it looks OK on the theater screen at the 48 Hour Film contest showings (with the contest producer's help).  There are enough things that can go wrong in a 2 day, 5 minute animation, I'd rather not add another potential problem.  Maybe next year, after some experimentation ...

Mark B: "there are times that render a scaled down version and then up-rez it in the video editor to soften the look a bit and decrease the Carrara (or any other software) rendering time."

I assume you stick with the same proportions that you will end up with?