You do not have to let the battery run down before you charge it. I charge my iPad every night no matter how much battery power is left. However, you don't have to charge it every night either if you have enough battery power left. There is no hard and fast way to charge. Use the iPad and charge the battery when it is low. It's that easy.
You can use the iPad while it is charging. I am using my iPad to type this and it is charging as well. You can read this about the do's and dont's of the iPad battery.