In reply to Dave the Rave:
Yes I'd agree with the above. You can get decently adjustable hoods that fit well either with or without a helmet. Much more flexible if your hood is over your helmet...much faffing if you have to take your helmet off to put your hood up or down. Not to mention safer to not have to take off your helmet.
Hood over helmet also leaves more air to insulate between the hood and your head plus a little more room in your helmet for ther headwear.
Just being fussy too I sometimes find a hood under a helmet can just feel a bit irritating as you move...especially a hardshell....but then the same can be true of the wrong hood/helmet combination worn the other way also!
For what it's worth I've got/had ME, rab and haglofs hoods that were all brilliant with or without a helmet. Marmot hoods for me seem to work best with a helmet under them or maybe I've got the wrong shaped head!
Hope this was helpful.