I think any shark that starts displaying aggressively means it's a good time to leave, if only the immediate area.
So far I've dived with whitetip reef, grey reef and scalloped hammerheads in the red sea, then last year went to Turks and Caicos where there were caribbean reef sharks on almost every dive, and one nurse shark we came across on a night dive. None of them were any trouble at all.
In seven weeks time my buddy and I are heading out to Egypt again, taking a liveaboard down to the St. Johns reef area, just north of the Sudan border. Apparently somebody recently returning from the same trip with the same dive company was quoted as saying of the area, "if sharks were aeroplanes, this place would be an airport", so I'm hoping for good things.
Luckily for us, we are then staying on for a second week, day diving from Marsa Alam. This means that we can dive the legendary Elphinstone reef. There are 3 large oceanic whitetips that frequent the area, and they apparently get their kicks by "bumping" the divers. But nobody has ever been bitten, or so they say. I'm looking forward to it, but with trepidation I must admit. Someone my buddy struck up a conversation with on Divernet (an English based forum), had recently dived Elphinstone, and saw 5 different species of shark on one dive, including silkys and threshers. The oceanic whitetips weren't there on the dive, but turned up later when some of the group went in snorkelling between dives, and began the bumping routine. Everyone decided it would be better to get out of the water than take any chances.
To be honest though, the thing that scares me most is the fact that we are going right in the middle of triggerfish breeding season, and if any of you have had the misfortune to swim into a Titan triggerfish's territory when it has eggs, you'll know exactly what I mean. The sharks give them a wide berth too.
David