Lower, go for your kill shot. If your opponent is even with you, use a passing shot. If your opponent is in front of you, use a passing or ceiling shot. When in doubt, go with a down the line shot as close to the side wall as possible. This should force a weak or defensive return.
Good angles are more important than hitting the ball low.
By using angles and hitting higher on the front wall, you eliminate skip balls or other unforced errors. Kill shots aren't the only offensive returns. A good passing shot that bounces twice before hitting the back wall will win a rally and is less likely to skip. Do not force your kill shots. Use your passing shots to get yourself in better position to hit kill shots.