panning hard left/right is a bad idea.
if you make a double mono trakc and pan hard left and right and apply haas you will have troubles with phase and comb filtering when checking in mono.
even though it's dual mono it has a sample delay attached to one of the channels.
if you pan 50/50 you still retain some of the information on both dual mono channels as.. umm.. one mono channel, and you won't have enough troubles with comb filtering etc.
just try it. you'll see.