It really depends on the rider position. In the pro peleton they spend an amazing amount of time and money on wind tunnel testing. These helmets can save about 10 watts at 25 mph. Does not sound like much but when 5 seconds can mean the difference between wiinning and losing it's huge. Every rider will have a different position on the bike so they have to find the helmet that works for that position.
A couple of interesting links with all the different helmets used in the Tour de Califronia and wind tunnel test.
http://www.active.com/cycling/Articl...California.htm