Priests. Why we are so ..umm.. difficult.

[Treesh]

because I assume Littledude and Sabramage have Improved Arcane explosion and won't be interrupted when AOEing with it.

LD does, but Sabramage doesn't yet (or at least the last time asked she didn't quite have the points put into it). This does make a difference on when to shield. As a result, Sabramage gets shielded sooner than LD does (unless I see him casting flamestrike or channeling blizzard).

I will certainly concede that you save this amount of mana, but I'm not sure that that amount of mana is truly significant unless you are in a long high pressure battle (in which case aoe is probably not your opening tactic) or you get a string of adds (which certainly CAN happen When Things Go Wrong™ but shouldn't be a common situation).

And these are the situations I use that tactic most. If LD or Sabramage is going to pull with AoE, then they get preshielded. Most of the time, things don't fall under the AoE pulling or the drawnout battles so it really doesn't matter which way I go since I've got a big enough mana pool and good enough mana regen.

I literally eat on average about one to two mana pots per week!  This includes both 5-man runs and raids with varying numbers of healers from one to three or more.

Still more mana pots than I drink. ;)

Even though your method will be successful almost as often, I see no reason not to get into the habit of preshielding every time because it has that small extra measure of safety and no true downside.
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Doing the same thing every time in every situation regardless of suitability of the tactics to the specific situation is simply lazy playing in my opinion (although I do play lazy like that myself :) ). It doesn't take much time to assess a situation and do it so you are covered when things go wrong.

Also, I don't get aggro by healing, whether I'm preshielding or not. I really don't know what other groups are doing so the priest gets aggro so frequently, but about the only times I ever get aggro with Aleri is if I heal GG when he's just trying to run through critters in the world (stupid reflexive action ;) ), a patrol comes up behind me so it sees me first, or there are simply so many critters around that they haven't all been hit with some kind of AoE aggro generator from someone other than me. So preshielding to avoid aggro is just as silly to me as not preshielding to save mana is to you. It basically comes out to be the same in the end. I'm not saying that not preshielding is the way to go in every situation. I'm simply trying to get some of the priests around who think of shielding as a "use all the time, everywhere on every class" skill to look at that tool in a different way.

There are situations where the timing of shielding is a difference between life and death for your mages. We were in (I think) Zul'Farrak and the plan was for Littledude to do an AoE pull so I preshielded him. It went down in a heartbeat and even with renews and spamming flash heals and reshielding, he died. A little while later we were in the same situation. I did manage to keep him alive with the preshielding technique, but I burned a lot of mana. A lot of mana because he was taking so much damage. With PITA patrols wandering around (I really hate having to constantly untoad or unstun people, stupid trolls), burning that much mana was not a pleasant thing so I thought of another way to keep Littledude alive and still be able to conserve mana. I tried it on the next similar pull we did and it worked to keep him very healthy and my mana pool healthy as well. It's also still working very well in our 4 manning of Sunken Temple. We are slightly overlevelled for it (52s and 53s in the group), but that's not too bad considering it's a 4 man rather than a 5 man group.

So yes, there are situations for both preshielding and not preshielding and it's up to the priest and his/her group to decide what works for them and when to do which. I personally like to try new tricks and new tactics for every new situation. If you can't adapt to changes, you'll end up dead in an MMORPG. The more tricks and tactics I learn now, the more ready I will be for any surprise changes to whatever class I am currently playing.