I did an experiment, using Poser 7.

I took the scene that I used to render my avatar, and timed the second and third renders at around 127.5 seconds. The first render was a second or so slower due to the loading phase taking longer. "My" glasses' lenses use refraction so they take a while to render.

Then I positioned a plane in front of the scene so it blocked off all of the camera's field of view. Renders now ran at around 21-22 seconds.

I made everything in the scene invisible apart from the plane. Renders now took 8 seconds or so. Conclusion: in Poser 7 at least, hiding elements that the camera can't see will cut your render times. It may or may not be worth doing according to other factors. Try it for yourself! I used svdl's timedrender script to tell me how long renders were taking.

I ran another render with the hidden parts deleted from the scene, i.e. with the plane only. That was about the same speed, so as far as the renderer is concerned, hiding an object is the same as it not being there.