I use rank 1 in normal dungeons, max rank in heroics. In normals, I'm barely taking damage, so I don't have the mana to support max rank. In heroics, I still get hit enough to keep my mana up, and having solid threat so that I can burn targets down without pulling aggro is a priority. You should be fine in heroics as long as you put a bit of work into finding blue gear. The Felsteel head/legs/gloves with +12 stam gems is a great start. If you load up with that kind of stuff (there are some nice quest rewards along those lines in Netherstorm/SMV), you should be fine for the lower heroics like slave pens.

Note: when you're still wearing blue gear, I'd use max rank in normals as well. You're still taking enough damage to get the mana back from heals.