What level are we talking here? Pre-70 there is a valid argument for downranking in many situations. Consecration and Holy Shield usually come first. Blessing of Wisdom may be vital.
At 70, downranking becomes not only less and less useful, but a heavy drawback. Rank IV Consecration retains most of your additional Spell Damage so it can be fairly efficient in situations places like Black Morass when you're running with ungeared DPS.
Heroic+ tanking mana conservation comes down to not being stupid. I've never been a believer in holding back on the threat potential. I should always be a few seconds away from going OOM if I'm doing things right. My method of mana "conservation" (perhaps "management" works better) is then to take more damage. Bigger pulls, sit down liberally (eat the insta-crit) etc etc. A heroic boss fight usually lasts long enough for me to deplete my mana pool, drink a pot, and MAYBE with slow DPS use half of that mana going all-out. I try to use every single point of mana that I can in a trash pull, the lower I end with the better. When time isn't an issue (Black Morass) I'll ideally be drinking after every single pull.
In the few situations where I want to reduce my mana usage (rather than increase my mana generation) the first thing I do is make sure the DPS understand what's going on and make sure they know kill orders solid. Rank IV Consecration should keep you ahead of the healers alright, especially with reactive damage thrown in. Then it becomes a matter of burning one target at a time with moderate DPS on them. Sometimes I'll forego judging (especially if it's the 2nd or 3rd target and already has a threat lead by the time the DPS gets to it). If it can be afforded, I'll drop Holy Shield. While the mana that Holy Shield uses is like a spitball on your mana pool compared to other abilities, it's threat:mana ratio is pretty poor and the actual amount of raw threat you can expect from it is significantly lower than even Consecration at the right gear level. Also, you take slightly more damage when you aren't casting it for better returns.
While I can imagine it being possible to get by with downranking in heroics if your group isn't stupid and follows directions well -- you really can't afford it in any kind of raid. Even in Kara -- taking more damage is a far better solution. You'll lose Attuman & Midnight or Rom and Jules pretty easy downranking, but you can glue them to you going all out and sitting anytime you have a free GCD and can't mess up your timing. Sticking to the "intro" raid (since I really don't know what level we're talking), Curator needs his own mana management (and many of the techniques used on him can be copied to Rage Winterchill or other bosses that hit like pansies). I tend to keep SoR up, judge occasionally (not every CD), keep Holy Shield up, and Consecration maybe 2 or 3 times per phase while unloading everything (with wings) on evocations and the enrage.
Most fights where it REALLY matter that you throw out huge numbers (to beat timers that are meant to be a challenge) you can rely on the boss dealing enough damage to support a full threat rotation though.
Edited, Aug 23rd 2008 2:34am by Losie