Hey Ross,
This entire depends upon your studio's environment. However in practice, if your scene data uses relative references as well as a well designed directory structure, it isn't a difficult proposition to do rendering on both platforms. However, due to differences in architecture such as a G5 versus a x86, we really don't recommend mixing a render specifically because of issues such as differences in floating point results. This was something we've seen in production where mixing SGI MIPS processors with 80x86 processor differences especially between 64 bit architectures such as MIPS and Sparc can produce enough of a difference to create pops between frames done with one processor and another. Of course, if you're using the new x86 macs, w/ x86 PC's then in practice mixing os's shouldn't be a problem.
Anthony
