Pixar does not make realistic movies. Their style does not need physical accurate shadows, just shadows which work in a movie. They use ray tracing and AO as rarely as possible, since this stuff costs much more render time - and rendertime is expensive. If an effect needs rendertime (remember Sully's fur in Monster's Inc?) they just do it and spend time and money, but only if the effect is worth it. Realistic shadows and lighting would not be that much obvious when you make toon stuff, so don't expect them to spend their money there. Pixars Renderman couldn't even do ray tracing for quite a while.

If I would do a movie with poser, I would try to use the P4 renderer wherever possible, because it's faster. If it's not possible I would avoid ray tracing and AO, just to shorten render times. I would use any trick I know to make scenes simpler, use pre-rendered backgrounds wherever possible, Stuff like that. The main characters would get the render time they need, but not more. And that's exactly what pixar does - spend a lot of render time for your main effects, but save it up everywhere else.
 

But if you want to make stills and/or need the quality, go for ray traced shadows and AO. You will need it.

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.