When I feel strongly enough about the subject of the song,the lyrics almost write themselves.
I can't just write words to fit a song,there must be a reason.
I've sometimes used the method you describe to work melody lines and to give the song some shape,but I never sit down to write 'words' for a song,just for the sake of it. There must be something to drive me,some anger,passion,joy or misery,otherwise the lyrics let down the song.
This is what works for me,there are plenty of great songs with rubbish lyrics,so there are no hard and fast rules.