It depends a lot on what your group can manage

We think one of the biggest challenges in encounter design for a 7-year old game is coming up with mechanics that long-time players haven’t seen a dozen times before. To that end, we're really reluctant to put too many constraints on encounter design. The majority of our fights have 2 tanks and we think that’s close enough. Shoe-horning World of Warcraft Gold a 2nd tank role into a fight for which it doesn’t really fit doesn’t make that 2nd tank feel any more heroic – they are often just soaking cleaves, doing taunt swaps, or corralling adds or something.

On normal mode Firelands, the numbers are tuned such that you can get away with comps with fewer tanks and healers, if you’re overgeared or fairly skilled. In Heroic mode, there is only one fight (Majordomo) designed as a single tank fight. Some guilds use one tank on Baelroc, but that puts a lot of stress on the healers, and we see a lot of guilds use two as well. We also see DPS sometimes try to tank, or see tanks tank in DPS gear. It depends a lot on what your group can manage.

I’ll also point out that we had a much more inconsistent number of tanks before dual spec was introduced. It’s not really a dual spec issue at all. We wouldn’t expect a lot of zero tank or five tank fights. But again, it’s more important for us to have diverse, fun encounters than for everyone in the raid to get equal time in the spotlight on every single encounter. If you never get your time, that’s another matter, but I don’t think we’re there. On the other hand, some fights (Shannox and Alysrazor) give two tanks some really fun things to do.

7 also isn’t a lot of bosses (in terms of pure head count, not content or quality), so it’s hard to extrapolate a lot of “trends” at this point.