when I run my five on one box I use the following:

Core 0 not used
Core 1 Main Wow
Core 2 Wow 2&3
Core 3 Wow 4&5

reason being that if the system needs to do anything it has core 0 available, Core 1 is for my main instance and the other four run on the two remaining cores.
It has worked well so far without problems (Vista Ultimate 64 8GB RAM)