What exactly do you mean by "align with z-axis"? The camera is aligned with the the z-axis by default. If you want to align it with the z-axis of an (rotated) object, there is an align()-method in Camera. If you want to align it with any vector, you can do something like this:
// Look up
camera.setBack((new SimpleVector(0,-1,0)).getRotationMatrix().invert3x3());
But it's cheaper to do the rotations directly on the camera instead of aligning it afterwards. Or did i get you wrong?
About the message: You are right. I've avoided this until now, because i don't really see it as an error. But i'll add a one-time warning once you are exceeding the limit (to not spam the console with such messages).