Some offenses don't designate a low post player. This can cause problems if you only practice with one or two players defending the post. If you have everyone defend the post at some point, as you should, your defense will have no problem adjusting to a 5 out set, 1-4 high set, or any other type of open-post offense.

Further Discussion:

The biggest thing you need your players to do is recognize where the ball is located right away. The second thing they need to do is recognize if they are one, two, or three passes away. This will tell them exactly where they should be in the defense.
It takes work to cover low and then get back out to the player you're guarding if there is a skip pass. Reassure your players there is plenty of time to make this adjustment if they move on the flight of the ball.

Rotation When There Is No Post