hum... that's interesting that that second site says to tie in the middle of baby's back if he/she doesn't have head control yet... I've been tying mine under her bum and she seems secure enough, I have a Babyhawk XT and it sort of forms a pocket. Of course I don't do anything particularly vigorous with her in it either... still, food for thought.
Though, honestly, I have not yet put the baby on my back with it yet. I've mostly been carrying her on my front.
Sometimes I tie the straps around her back, sometimes around her bum. It really depends on what she is doing. If she is nursing, I tend to tie it around her back more because she is down lower on my torso. If she is sleeping I tie it lower because I tend to carry her higher, if that makes sense? I've found that the carrier provides enough head support because mine is curved up at the top - I think the Baby Hawks are like that too.