It has for me nothing to do with the tool you are using. You can start to work on arts but better not the final ones and do not integrate them in your game prototype. Why?
Just because if your gameplay is juicy and fun with placeholders, it will be even better with nice looking graphics. It is already difficult to judge his own work and decide where the gameplay can be improved, parasiting this critical phase with nice looking graphics would make it even harder. It does not prevent developers to have placeholders with right sizes and collision boxes. And if you are using procedural animation tool, you can even use the same animations in a later phase of prototyping.