Designating Songwriting Credits

...my understanding is that the musicians don't get credit for simply coming up with their respective parts.

however, i also understand that if the bassist or drummer invents a "signature" part that is deemed integral to the song, they should receive credit.

where it gets cloudy is, for example, if the bassist suggests that the chorus should come after the bridge, or if he suggests an F#m in lieu of an A major.

how do other songwriters deal with similar issues?

-dh