Also it will affect all playthrough of SP.
How will it affect SP?
there is a war asset in SP, minerals, people saved,alliances etc. in MP there is a factor called Galactic Readiness,this is multiplied to the war assets to help you to get a better eneding, GR has a decay to keep people playing online,but it is permanent on all SP playthrough (from what people mining the demo gathered), so unless you want perfect ending never needing mp, dont touch mp.
Also from achievements, MP unlock things faster like, 2 mp missions equal 5 or 7 SP for teh same effect, so completists will have to be very througly on their planets scanning.
more here. http://blog.bioware.com/2011/11/04/my-interview-with-mass-effect-3-producer-jesse-houston/