1999 Pontiac Grand Prix Oil Burning

Hello, I have a Pontiac Grand Prix that I believe may be burning oil, at first we could see that oil was leaking from the valve cover gaskets so i've replaced those. When I start the car up it smokes, after it warms and I shut it off and restart it doesn't smoke, I've had an oil change done recently and had it checked, I was told that there were no signs of oil leaking and I've also had the engine flushed which was recommended and was told that was one of the cleanest engine theyve seen for that many miles. I have an oil reader in the car and when the oil change was done it registered 100% that was done 10/30/2010 now today 11/09/2010 it registeres 96%, is that normal and and what could be causing it to leak if there is indeed a leak?