As shellyj commented it is a tad uncomfortable when you first start correcting yourself, especially if you have a stretching issue in the calf. And there is nothing really wrong with riding this way, you just aren't getting the most out of your energy.

The other thing to try if you are using clipless pedals is to feel yourself actually pull up on the pedal. If you are riding with toes down you will be pushing more than pulling on the pedal.

kfergos- It is all about efficient use of the energy you need to pedal. Hopefully someone will give a much more technical explaination, but the essential part is you use less energy pedaling with you heel down.