I'm doing a mixed bag here. I'm on a PvP server so sometimes you just can't quest. 70s that used to leave my solo characters alone always seem to wanna obliterate my lvl 20s....

What i'm doing is running my characters through at the earliest level they are able to zone in, and basically farm boss loot, and collecting greens to DE/upgrade/Sell on AH (Master Looter FTW!) And once the group is level appropriate, I run the instance normally. It seems to work pretty well, because I end up being pretty well geared. I did many a VC run, got just about every item I could think of on every character (easier on my 5 man mixed class team than my 3x locks lol) and when I was 22 I ran it the normal way and had way more fun than when running with my 70. The added intensity of being vulnerable added to my excitement. I also boosted through SFK, though not as often, as I'm Alliance and SFK is a long way for lvl 10s to run lol, and ran at lvl 24-25 but due to all my gear being red, I couldn't take down the final boss, and didn't want to bother hearthing one by one to repair and re summoning.

I am now farming SM Library for with my 3 locks and another pally (who will be used to boost the chars on the account with my current 70 boosting pally) and gathering rested xp for my mixed 5 man team. Both teams are 24-26 now, so I may experiment with swapping one out. My locks do plenty of damage, but can't rez unless i remember to SS a pally, and can't off heal. (I made a "oh crap" macro that has all chars that can heal cast a heal on my tank, helps a lot for many "oh crap" moments) I have a few more runs to go before they all have the gear from Doan, then it's off to Cath (and I guess GY, but I skipped it as the gear isn't as great, but there's always the rares, and paladins PWN undead) Then I will boost the other team and farm gear, my pally should be fully decked out in Scarlet Chain stuff by the time he runs it as the tank. It's way more fun to play at a slight advantage and own, than play slightly too low/undergeared and wipe a lot (hence my inability to take down Arugal in SFK with red gear)

I will probably also quest a bit as well, but when avoiding collection quests, and playing 5 characters, you tend to blow through available quests before you are able to pick up the next part of a chain, or level out of a zone. The strength of 5 boxing comes in being able to do dungeons by yourself, which easily makes up for collection quests, and gets you some nice gear to boot!