In the old days you could land wheels up at any speed. HTC changed it to 150 mph at one point and I believe thats where it still is set.
ANY landing fully ON the concrete is a successful landing. One wheel off and its a ditch.
Personally I've always played the game as a cartoon pilot flying in a war (and I use that term VERY loosely). So to that end, I try to land with my wheels down on the runway. I try to fight my way into the battle area, accomplish my mission, and return fighting my way out. All that fighting with my limited skill doesn't leave open too many chances at landing, but I do ty to do them right when I get the chance.
To me, when you land a plane, its with wheels down. In the case of the game I'd be happy to see those that land ON the concrete with wheels up get a ditch only because they didn't do it right, instead of giving a bonus to those that do it right. If you land short of the runway because you ran out of fuel, you get a ditch. The mistake you made was not saving enough fuel to land. The same should go with wheels up landing.