If we think about hitting the ball with solid contact, what we really want is a line drive. Hitting a line drive is hitting the ball square, well as much as possible with two round objects. So if the goal is to get a player to hit the ball with the best contact and the biggest margin of error than the goal should be to hit line drives, not ground balls. If a player hits slightly under the center of the ball in that situation, then the result could be a home run. If the player hits slightly over the center of the ball then the result is a hard ground ball. Either way shooting to hit line drives will give a player the best opportunity for success and as coaches that's what we want to help them do.
Where to Focus
This is something where you need to watch a player hit in batting practice a few times to see if there is a tendency to consistently swing under or over the ball. If you see that tendency then it's time to discuss with the player what he is focusing on as the ball is being delivered. Many players will often just be looking at the ball and not a specific spot on the ball. A change to be more focused on a particular area of the baseball can help a player be more consistent and provide a side benefit of him picking up the baseball earlier out of the pitchers' hand and seeing it more clearly.
So if a players is hitting ground ball after ground ball, talk to them about where they are focusing. If they are just looking at the ball have them focus on the bottom of the ball. See if that results in line drives or fly balls. If the result is fly balls, then have them adjust and focus a little higher. Again the result in batting practice should be an increase in line drives.
Make the adjustment the opposite way for players that are hitting fly balls all the time in batting practice. Have them move their focus up on the baseball until the result is more line drives.
Either way the adjustment should provide the player with better focus on the baseball and better results.
I hope this is something you can take into your winter workouts and/or your spring practices as I believe there is no downside on working with a player to refine their focus when seeing the pitch being delivered.