hanging low will usually mean that its harder to access the low strings, if you stress it might even damage your wrist permanently. So if you wanna sling your guitar at your knees its ok if your playing simple chords, root notes etc.
But for some people with long arms and legs say paul gilbert or the nirvana bassist then its more natural for them to sling it low.
having said that i still sling pretty low coz it looks cooler. as for slash, when he solos he bring the guitar up into a vertical position to help easier access the frets. its harder to play that way, but who cares...its cool
and the punk rockers = sling low, hair metal = sling low, progressive = high, well its actually kinda true la
can you imagine the blink 182 guys with their guitars up their chest or dream theater ppl slinging their guitars to their knees?