I was always under the impression that any solid hit to the nose would cause bleeding from just about any one. I'd work on getting hit in the face less. Less hits to the nose=less nosebleeding.
I'm new to having sparring partners and while I'd never until recently had a busted nose in a fight, it turns out that if you spar 10-15 minutes with me my movements become predictable. I slipped almost every jab, then folks started jabbing where I was gonna be instead of where I was. I moved right into a hell of a jab that busted it up right nice. No break, plenty of blood though. Just remember if it happens in a fight, don't blow it out. I don't know why..but trainers always tell their fighters that in the between rounds on TV.
Bottomline..I had always accepted that busted bloody noses were a very common thing in boxing and that everyone ends up with a broken nose eventually. I wouldn't sweat the nosebleeds too much unless it's bleeding anytime its touched.