The DLC does add some War Assets you can collect for the readiness rating, but if you MP with us you can get the best ending since the % boost multiplies by the single player rating. If you get to 100% readiness you are doubling whatever points you get in single player once the final mission comes down.
Here is an explanation on how it works.
Here is an explanation on how it works.
So the way it works is that there are two systems, one is Readiness which is a percentage and the other is your Galaxy at War score. Both combined give you a number that calculates along with your choices what ending you get. You need a score of 3750 to get the Master and Commander achievement and a minimum score of 4000 to get the best ending. Now your Readiness percentage starts at 50% which is not affected by playing the single-player. 50% seems to be the absolute base in that it can't go below that and multiplayer makes end game much easier. Readiness can only be affected by playing the multiplayer. So to get the perfect ending, you have to get a GAW score in single-player of 8000 and have certain correct choices. In order to negate these choices you have to get a end game GAW score of 10,000 (or 5,000 once you calculate in Readiness). The bad news is that there is a decay system attached to Readiness which is linked to real-life time. However note that Readiness cannot go below 50% and is so the decay will only affect the bonus from multiplayer. Something like 1% a day.
What this means in regards to the endings is that to get the perfect ending, you have to play through the singleplayer game and do EVERY sidemission to get the perfect ending. Now you can do multiplayer so you have to do less sidemissions but that bonus will decay in real-time. You can take your time but you'll have to play the multiplayer or the iOS game to maintain or increase your readiness bonus percentage.