I didn't read through this whole thing yet, but I like what I see so far. I have been 5-box boosting instances with an 80 for the most part and might try the non-instance quests a bit for a nice change of pace. If you want a decent instance grind 5-box, try a WoWHead thread I made back in January and let me know what you guys think:

5-box Recruit A Friend leveling 4 toons with a 70+ (quests to get)

As I am reading through more of this thread, I see that ELBA was trying avoid the kind of writeup I did (like I said above, I have a 70+ boosting) so I thought a "compromise" of sorts would work:

If you are 2 or 3 or maybe even 4-boxing the instances would be nearly impossible without outside assistance. But if you have *at least* 4 and you waited until the quest(s) inside each instance was on the lower end of yellow or green it would be very do-able. If you have a decent grasp on how to handle all 4/5 of your characters, you could fly in and do all of the quests with ease if you waited for the right level.

Just as a guess, here's a listing of when you might try the quests I listed (I just went to each quest and looked at what level the quest turns yellow):

RFC:
~14 for Searching for the Lost Satchel
~14 for Slaying the Beast
~14 for The Power to Destroy...

SM:
~36 for Compendium of the Fallen
It would go against my guide a bit, since I always move to ZF at 40 since I'm boosting, but...
~40 for Into The Scarlet Monastery

ZF:
~43 for Scarab Shells
~45 for Divino-matic Rod
~48 for Gahz'rilla

LBRS:
~57 for The Pack Mistress
~57 for Operative Bijou (and all the followups)
~58 for Mother's Milk
~58 for Urok Doomhowl

After doing the above, I am now even thinking of checking into the instances I normally don't bother with while boosting and adding them in here. If I decide to just level 5 at once instead of boosting I'll try it out and see what comes of it!

EDIT: I want to add that I was slow (*am* slow really) at the 5-boxing for a while, but now that I have gotten myself a little more polished at it, I can get 4 toons to 60 boosting with my Mage in about 1 day 6 hours, and probably less. I have a BAD habit of looting when I shouldn't bother, so I could likely get 4 to 60 with barely more than, or less than, a day played.

P.S. Sorry for the many edits. I do that a lot when I type something and don't read it through well before hitting [Submit] 8|!