I say do it either way, whichever comes more naturally.
If you have some killer lyrics come out of you write them down keep them around and if you happen to have some music to go with them put the thing together into a song.
Or you could write lyrics to a song you have written already.
I have done it both ways. Works either way. But best if you have a theme in mind for a song musically and lyrically ahead of time so you get the feeling right for both of them.
