With Lesson 10, all your single songs will be good. Echo Song will be useful. But if you don't mind buying a Neuralizer to get rid of it in the future, then go for it. Just remember, Voice Lessons is just like Music Lessons? (is that the bard version of Dance Lessons? the second lvl one) the bonus will go up with higher lvls of this. 5 is just minimum to play a third skill with a second skill, like strings, up. Windmill is just great for those rangers who don't have WW set (weird but it can happen) and for SC's with AS. I suppose Echo is your next good choice. I'd personally get Siren's Voice, so if it gets super bad, you can just save yourself and get monsters to not aggro on you. The downside, is that it'll run on your party members.. haha.
So yeah, Echo Song should work for you if you'd like. The def would be nice. So it'll be your choice to buff atk or def. =)