I have disassembled my Promedia 5.1's to just 2.1's for the time being, I don't really need it that loud.
But referring to a couple weeks ago, I had my rears set at 8, because they flowed more seemlessly with my front sats, and I didn't want them to be the main focus of my attention (that's the whole point of surround sound, you want to be immersed in it but not really know where the sound is coming from). Center, I kept it at 0, and my sub, depending on how loud I was at (60?), usually at 0. I didn't really need to adjust it, because my SWS sounded better at louder volumes than the PM5.1 sub.