Cowriting is fun!.. so is writing alone. When cowriting, regardless of percieved input, credit should be split evenly (as if people are making money:)

As said above, egos must go. I do have one selfish rule though: If I'm recording the vocal, I will tweak (not twerk) the lyrics for phrasing purposes. I hear so many people on this site that have great lyrics only to fail with the delivery. It can't just look good on paper, it has to roll of the tongue of the person singing it without it being akward. I've even heard these problems on professional demos.

-Mike D.