Poser 7 does not appear to be doing so. It appears to be rendering in 4 parts.

My answer to your original question would be: yes all the 8 processor threads of a quad-core processor are being used during a rendering, but only 4 of those threads are spawned by Poser. The processor spawns extra threads to get the job at hand (assigned to it) done a little bit faster. A better rendering engine would take control of all the available threads and use them... now it just leaves lots of computing power underutilised.

As said by ghonma above, a software that spawns 8 threads would appear to be using all threads at utilisation levels nearing 100%.