It's not like the ending means a lot, but if you do care about it I think sooner rather than later would be for the better, because Galactic Readiness drops a little for every day you don't play MP.. I wonder how long anyone will keep playing MP.. It kind of marks the game as being very contemporary with such a connection affecting the game.
Meh. Everything can be edited. Either with memory manipulation tools or by editing coalesced.bin, MP is not necessary.
Didn't get the Valiant in MP so that's it for me. I did discover that with Gibbed i can have Level X weapons in SP plus Illusive man eyes so that might be extra incentive to play.