I'm assuming that the endgame save or final save or whatever you want to call it for Mass Effect 2 is created after your decision regarding the Collector Base.
What I wish to know, is: will it re-save with new information based on any DLC I complete?
For example, if I finish the main mission, and then proceed to finish the Arrival DLC, will my final save be updated so that in ME3 it knows I finished the Arrival DLC?
The reason I ask is because I know there could be some effect in ME3 based on what you did in DLC, so if my final save (I'm almost there) doesn't update, then that means that I'll want to complete all of the DLC before taking the fight to the collectors.
Best Answer
Every Mass Effect 2 save file has a "QuestCompletion" variable that can be one of three values. Gibbed's save editor has dubbed these, "Quest Incomplete", "Lived To Fight Again", and "Went Out in a Blaze of Glory".
There is no "mission complete" save generated by ME2 like there is in ME1. Instead, every save file has this variable in one of the three states. Presumably, only saves where Shepard "Lived to Fight Again" will be viable to import, as Bioware has previously stated dead Commander Shephards won't be importable into ME3. (And obviously, you can't import without beating the game).
(Related: https://gaming.stackexchange.com/a/52493/336)
When importing into Mass Effect 3, you get to pick the actual save to import (i.e., quick, auto, or manual). Your save is always current, so there should be no worry about keeping your save "updated", other than simply saving normally from playing the game.