Looks like the general concensus is comfort. So go for try it. If you don't like it go muzzle up. Besides the whole idea of a sling is comfort anyway. Someone a long time ago figured it was more comfortable to sling a boomstick over the shoulder than it was to carry it around all day.
As far as the concerns about fouled barrels and damaged crowns just as many concerns could be voiced about muzzle up. For example, when I as in the USAF I saw one young cop fall on his M16. He was running with it at port arms when he tripped and fell on it. Bent that barrel good!!! My point is it doesn't matter if it's muzzle up, muzzle down, low ready, port arms, or locked up in the truck. Freak accidents can happen. Gun safety, muzzle awareness, and do what you can to minimize the chances of Murphy's Law rearing it's ugly head.