Warm Up Engine Before Changing Oil?

I was discussing this question with a co-worker of mine the other day, and he seems to think that you shouldn't because the oil will get pumped to the top of the engine and you won't get most of the oil out of the block when you pull the plug. I seem to think that you should, because the oil will be hot and it will flow easier.

I'm curious to see who's right in this situation.

Jason

Says in service manuals to warm engine before oil change, true when started oil will be pumped to top end but will drain down fairly quickly, as opposed to draining cold oil, i have done both methods (cold, after doing other engine work) and it takes quite some time for the cold oil to stop draining, as opossed to the warm oil.