Yeah that's how I originally had it, but I changed it for 2 reasons.

1. I think we're leaning toward treating the circle as the default avatar style, so it's nice that the stylesheet reinforces that.
2. If we pass $isRound as an argument, then every 'square' avatar class gets its own style block in the CSS output, which takes up much more space than just adding a border-radius-killing modifier on top of the default base class.

From a CSS filesize perspective, it would be best to have square as the default shape and add a 'round' modifier to get the `border-radius` styles, but that would require some refactoring of the existing avatar markup and would work against # 1 above.