Personally, I've been finding even less than 3xBB to work just fine once the blinds are a significant size relative to stack sizes. 2.5-2.7xBB seems to work just fine. I'm playing on PokerSharks though where raises in general are more effective and players are more often weak/tight. I can't say that the 2.5xBB blind steal got all that much respect during the most recent FTR Showdown though. Of course, everybody there knows that everybody else is willing to attempt blind steals with crap and therefore are willing to defend their blinds more readily.