Pereferably, you should update add-ons everytime they come out with a new verison, though it is possible to get away with every patch without problems. The more updated your addons are, while they might take a bit more space up, thus making your computer run a SLIGHT bit slower, it's better then the program screwing up, and screwing up your entire UI ^^;
As for Add-ons?
Recount is my favorite for tracking things, Damage, healing, dispells, Ect.
Healbot is my personal favorite Healing Add-on, while others like Grid, though I've never tried Grid.
The third main one I use is Deadly Boss Mods, when Updated right ,this mod lets you know when bosses, that have significant abilities in heroics, or timers on enrages, and such on Bosses in raids.
As for other off the Wall Add-ons, for Roleplayers, theirs Flagrsp
For Gatherers, there's Gatherer
There's also Atlas Loot for looking for specific items~
There's alot of helpful addons, I usually have about 3 or 4 running depending on what I'm Doing, it's kinda hard, In my personal Opinion, to narrow down 3 I'd always use, because there's many that are helpful
I hope I helped ^^;