I have done it both ways and I'm not saying it can't be done just that i find it more restrictive. A lot of that has to do with the fact that melody comes very natural to me, where lyrics do not. It's good to hear different ways to approach songs and as i start writing a little more country styled songs i see more songs that are written lyrically first. I understand every song will go through changes to be good, I'm just that guy who needs thing proven before i say yes to anything. I take my craft seriously and I have always found for me at least that it's easier to find a lyric change then a melody change. Simple math logically tells me there are more words then notes. To me there's always another way to say the same thing but there's not always another great note for a melody.