It all depends on the project. If you are doing a single image, it may be worth the extra render time in MentalRay. But then again you can come up with some very nice renders in Maya using a bit of raytracing and good lighting. For animations it would depend on if you really need those effects or not. There are other renderers out there to fake them and they may be quicker, but the quality is lacking.

Let the project dictate the workflow....sometimes it's about meeting a close deadline and sometimes it's about taking the time to create the highest quality images. There are no rules set in stone other than do whatever needs to be done

Darkon