Defensive is certainly the defacto stance to be in, but the other stances have some benefits. The 10% damage reduction of Defensive is huge, as is the threat bonus.
The first thing to realize is that without Tactical Mastery you'll only retain 10 rage when switching - so you have to balance whatever you're doing against that; if you take a huge rage hit just to pull off a move not available to defensive stance, it might not be worth it.
Apart from that, the things I find useful outside of Defensive while taking are:
1. Hamstring. Although it's nice to hope at least one of your DPSers is awake and using a slow ability, it doesn't always happen through sub-60 content
2. Mocking Blow. If a taunt is resisted or you needed to grab two mobs real quick, this ability is key.
3. Charge. I always try to charge if the pull allows for it. It's free rage that you can immediately dump into an Imp Thunderclap before switching to defensive.
4. Execute. For bosses this is a pretty sizable DPS increase as long as damage mitigation and healing are in the green.
5. Intercept. Near-instant travel ability. You're a melee class, you want to be right up on it hitting it, not slowly running towards it!
6. Berserker Rage. Used to dodge fears mostly, but I also try to hit it on the rare occasion I switch to Berserker stance (after Intercepting usually; although you have to judge whether you need your global cooldown for something more important.)
Apart from that, the majority of time ends up being Defensive stance.
Also..keep in mind my warrior is only 60. While I'm not an expert on endgame tanking, I'd like to think I did pretty well with these tips while tanking through pre-60 content in Arms (which is exactly what the OP will end up doing.)