If you have like... 10 cores... and you use Apache and your load average is always under 5.0...
Would it slow your site any if you lowered to 6 cores?
(assuming you never reached any spikes that hit 6.0 in load)
Specs: Xen, 10 Cores at 2.0GHz, CentOS 5 64bit, using "uptime" to find load averages.