According to the Polymer docs, the WebComponentsReady event is necessary because...
WebComponentsReady
The polyfills parse element definitions and handle th
In Polymer 1.0 you can just listen for WebComponentsReady.
In Polymer 0.5, because it does more things asynchronously, there's an extra event called polymer-ready which will fire when your elements are loaded. Here's a jsbin showing the order.
polymer-ready