http://www.answers.com/topic/lyrics -> Even though nothing is absolute, the history of the word and general knowledge of poets and poetic societies agrees with me.
My songwriting process: Write until I feel like I'm done. I start from anywhere in the lyric most of the time, varying from song to song. I don't think up a topic beforehand most of the time because I'd rather let my subconscious do the thinking for me. Most of the time I don't revise, unless an area feels weak or redundant. Going to music, I usually have the guitar part written up on an acoustic, and either scream (if I see the lyric as being more of a full band thing) or sing (if it's going to be solo/duet) the words into the song. Works out fine for me, but most of my lyrics just remain poetry.