If you have a somewhat clean lie, it is easier for most players to hit down on the ball and achieve crisp contact with the ball further back. The ball will fly at a lower trajectory.

If the ball is in thicker grass and/or you want to hit a bunker type shot or flop the ball high in the air, place it forward. You will be more prone to skull it, so be careful.

In short: back is safer, forward is cooler