It would also make a difference if the axis of the drain hole/plug is vertical or horizontal. Obviously, if it's horizontal, the wall thickness of the valve would prevent the last little bit of oil from draining - not sure how important that is because even without the valve, there is some left in, just not quite as much as with the valve in that case. If oil is changed regularly, 95+% of the junk will be suspended (floating) in the oil and will come out anyway. "Chunks" or settled particles could get hung on the lip and stay in - but again, how much of that will there be with regular changes.
Also, because of settling particles, it's good practice to run the engine at moderate speed a few minutes immediately before removing the plug (or opening the valve) to stir everything up and hopefully get most of that stuff that settles. The theory is that if it's *THAT* resistant to coming off the bottom of the pan, it's probably not going to get pulled into circulation while driving, and if it does, wouldn't the nice clean filter that's changed with every oil change catch it?
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")