I think that's all rather personal preferance and/or common sense. What good does running MikScrollingBattleText do on slaves? Do you really need Auctioneerx5? Do you need really need an alternate map on all your toons when you drive form the main? Do your slaves really need custom name plates? I could get by with Pitbull only on my main and my healer, but I'm not sweatin' the resources, so why not run it on everyone?

I just load up a resource monitor like sh:Performance and look at what the various addons are taking up. Aloft was a huge pig, so I dumped it in favor of Tidy Plates, which only my main needs. FuBar was a waste of resources, when I can just let Bagnon handle my docking. Gathermate and Lightheaded were a tie for "OMG WTF?" so Lightheaded got the boot and Gathermate runs only on my main. And I use Addon Control Panel to enable and disable mods on a need-to-use basis. Things like Gathermate, Silver Dragon, Questing Mods, Tradeskill mods do me Zero good if I'm just using the dungeon finder to auto-port from Dalaran to the dungeon and back again.