Most of the time I have MP turned off. Most of the plug-ins that I use are not MP aware and I seldom use AE's render Cue for rendering for delivery. I use AME and keep working on projects while I'm rendering.
Basic rules, at least 2GB per processor, at least 1 processor for other apps, Make sure the CPU's actually used matches the CPU's assigned, then test with your most common project. If you are doing something tha bogs down the system just turn it off.