Any time I've tried otherwise, it's failed. I'm noticing that when I let me stream of consciousness lead in a song idea, it's a melody. I may have a single lyric line that sparked the idea for a song but I find that I'm now led by a vocal melody and the rest falls in to place for the music.
I should say that I've mostly stopped trying to write 'riff oriented' music. I am going way off the hard rock path nowadays so it's been a journey but very freeing.