Here's the scoop for my shammy team leveling. I instance powerleveled them until L40, and then went out into the fresh air with questing. I followed Joana's Horde Leveling guide very roughly, thinking that I'd be OK with my progress if I didn't do every quest (especially collections) or do some of the hopping around. I was also quite aggressive in the level-range, since my three little shammies could kill things multiple levels above them. Worst case, I could add my L70 to the team and blast out any super tough quests. (He was on auto-follow, ungrouped, because he tended to scare off world PvP)
Long story short: I have gotten "ahead of myself" with questing. Since I blew out most of the available Un'goro quests at L48 instead of the recommended L50-51, I was forced to move on to Felwood. Now, most of the available Felwood quests have been done at L51-52 instead of the recommended 54-55, and I'm shopping for the next zone worth of quests. I'm continuing to be able to kill far above my level due to Totem of Wrath +Hit goodness, but I'm starting to get concerned that I'm meeting fewer and fewer of the minimum quest level requirements and will further exacerbate my problem.
Since the lowbie team is just a group of 3, running my own instances is less of an option than if I had 4/5 shammies. I think I may be forced to grind mobs for a level or two in order to activate the L55-58 series of quests in the last Azeroth zones. Fortunately, I have no plans to stick around Azeroth from L58-60, so maybe the "problem" isn't as bad as it seems.
Anyway, moral of the story is you should be a little careful to not pass on too many collection/problem quests if you aren't instance grinding as you level up your multibox team. It's possible, even with as many quests as there are in pre-BC content, to get ahead of yourself quest-wise. I blame Stranglethorn Vale, which I largely skipped since I hate that zone with a passion of a thousand burning suns. Moving up to the next zone of quests is probably what started my problem...
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