Jen
KurJen - Kurgan Mod
MP can help you make up for small oopsies during the campaign basically. It is not necessary as enough War Assets are available in the campaign to get the "best" endings available. Just to let you know appearantly there is a certain total you do have to reach and as you progress, if you aren't racking up the Assets quickly enough, you might wanna turn to the MP.
I can see that....I always made sure to do every single side mission rather than just doing the main ones and making the story progress. The last time I played, I was able to be 100% paragon before the Suicide Mission so I was able to use the paragon conversation options to keep everyone loyal. I would imagine this would be the same way...if you do every little side mission successfully, you will gain the necessary assets for the optimal ending.
Don't think this is a spoiler since I've seen it in the trailer - but I will put it in spoiler tags anyway.
One thing I keep reading and seeing in the trailers is how Cerberus is now an opponent...I would have thought if you had given them the human Reaper info at the end of ME2 you would still be on good terms. I'm glad I didn't give it to them, especially if the are no longer allies. And I'm glad I've got good relationships with all the alien races...except the Batarians during the Arrival DLC...but that could not be helped..I did try to warn them though.