It really depends on who's yoga philosophy you are following. In the Iyengar tradition, they would say, knees straight (not exactly locked, but "engaged") and if your heals are off the ground that is just fine.
The idea is that you have a straight line of energy from your hips to your heals and from your tailbone to the top of your finger tips.
The other thing to keep in mind, especially with your belly/bolly dancing training, is that you want to draw your abs in. This really takes the load off your back. You really should not feel any pressure in your back at all.
I hope that helps a little. Downward dog is one of those incredibly complex/simple poses. Learning how to do it well actually takes lots and lots of practics, so I have finally come to accept.