Hi, David !
For me it is always the music that comes first !
I think it is easier to adjust the lyrics to
the song when it is as good as ready, but
the ideal seems to be that you can work
on both at the same time i.e. you can always
change either the music or the lyrics
to get maximum flow !
I find it more difficult to try and write
music to defined unchangeable lyrics !
Cheers
Dani