Power supply design for CPU: Define maximum current needed
Design for worst case and add some derating to keep on the safe side depending how hot the ambient can get etc. Never ever design something with the mindset "hey, we're using 5A current so I'll use 5A parts, m'kay"! 

There's IPC-9592B standard which is squarely aimed at codifying how much derating you actually need to be "safe" or at least it gives you something to refer to when the house burns down and investigators come calling. 

Douglas Alexander's website has a derating table but these are NOT IPC standard, it's just something he's come up with experience. I don't obviously endorse them for any commercial use but it gives you an idea what a proper design margin would look like.

http://www.componentsengineering.com/procedures-guidelines-2/component-derating-guidelines/

http://www.ebnonline.com/author.asp?section_id=1541&amp;doc_id=235329