It seems to me that everyone is missing a fairly obvious reason to have the valve at the top. That tanks are rounded on the to and would therefore move through the watter easier. Admittedly, divers are generaly not moving at speed through the water, but given the increased density at depth, I for one want as streamline a profile as I can get. I would think that the base of the tank would create a large drag while swimming.
If you look at all the pics of upside down tank configurations, they all had some way to stream line the profile and most if not all used double round tanks.
For the record, I know that firemen use upside down tanks, but I believe they are double rounded as well.