really i think it is completely up to you. I personally use both. When you write a song, you should already know what it is going to be about, and what emotions you will try to convey. In that sense it may be easier for you to write music around lyrics or the general idea for the song. However many people find it easier to write a song and then, knowing how the song sounds and what the emotional appeals of it are, write lyrics afterwards. Both work but not for everyone. find what works best for you