A little bit of history
Most of the taunt-hit-confusion arises from the various changes that have been applied in this area over time. Growl (and the other taunts) are classed as spells. Before WotLK, there were 2 types of hit-rating: spell-hit-rating and "regular" hit-rating. The regular version applied to physical attacks: melee attacks and hunter shots. Spells and physical attacks had a different cap: back then it was 17% for spells and 9% for physical attacks against raid bosses. Growl was affected by spell-hit-rating and you'd need 16% spell-hit to cap Growl (back then you always had a minimum of 1% chance to miss with spells).
In patch 2.3, Growl was changed to be affected by regular hit rating. However, the rest of the mechanics didn't change: You still needed 16% hit to cap it (keeping the 1% minimum miss chance). The only change was that instead of spell-hit-rating, it was now regular hit-rating that did the trick. This was useful, of course, since regular hit-rating was found on tank gear, where spell-hit-rating was not.
With patch 3.0 and WotLK, both flavours of hit-rating were merged into a single stat we now know and love as hit-rating. However, Growl remained a spell with all its properties. And even though both spells and physical attacks use the same hit-rating now, you need more hit-rating for 1% physical hit than you need for 1% spell hit.
So how is it now?
To never miss a Growl, you need 17% hit. You need 26.23 hit-rating for 1% hit with spells. You can glyph for 8% (210 rating), get a Draenei aura for 1% (26 rating) and get a Shadow Priest or Moonkin to debuff the boss for 3% (79 rating). So that leads to the following numbers:
|Glyph/Buffs||Rating needed for taunt-hit-cap|
|Glyph + Moonkin/SPriest||157|
|Glyph + Moonkin/SPriest + Draenei||131|
The thing you can control yourself is the glyph, you can (and should!) swap this one in for any fight where a successful taunt is required. With the glyph, you need less hit-rating than you would need for the regular melee cap of 263. In a 25man raid where a Moonkin or Shadow Priest is pretty much garantueed, you can quite easily hit-cap on Growl already.
Due to the Growls spell-character, you don't need to be at the regular melee hit-cap to never miss a taunt. With the glyph, a lower value suffices. I would like for Blizzard to fix these oddities in Cataclysm, they seem to want to make stats easier to figure out and this is one area where some shrouds of mystery can be lifted.