Flat wrist at the top is not important. What's important is what you feel more comfortable with. I use the interlock, that's how I first learned. When I make a swing using a overlap or 10 finger, I can't even hit the ball. Use what grip you have now, because that's the grip your swing is used to. I think the interlock is the better of the grips. It's not the most used, but it promotes a better hold on the club, which resists twisting of the clubhead at impact.