First of there's one thing we have to make clear, this rotation is NOT about building threat (enmity) if you're having trouble building threat you do as many BB Combos as you need until you have secured your top position as tank.

This rotation is about maximizing YOUR DPS and your raids DPS (10% Slashing) while still maintaining enough threat, it's mostly aimed at 8-24 man boss-fights.

Personally I mostly only have to open with 1-2 BB Combos. After the opening of the fight you always have to make sure that you're ahead when it comes to threat, there are fights when you're not gonna be hitting the boss for a moment but the healers are still gonna be healing (causing threat). It's YOUR job to know when this is and how you handle it (mostly BB spamming gets you back on top). BB Spamming is NOT however the most efficient way to tank the whole fight, unless you think tanking is only composed of taking damage and not actually contributing to the fight in any other way.