Obviously, it depends very much on the distance from goal. If there's a free kick from 30 meters a low shot will be very inefficient because of the friction force with the ground; the goalkeeper will have no trouble catching the ball.
When there's a closer free kick you have to consider some factors:
- Kicker's ability
- Wall's players height
In the first case, the defending team should be aware of kicker's habits. Most of kickers usually maintain the free kick style: David Beckham has always kicked the ball making it fly over the wall; in this case the wall has to jump. When the kicker is someone like Ronaldinho, the wall strategy becomes a bit harder. In my opionion the best way to avoid a score is to make defenders jump in an alternatively way: I mean, if you have a 5 defenders wall, you could make the 2nd and the 4th jump, and let the others stand still. In this way you should reduce (theoretically speaking) the probability to get scored.
In the second case if the defenders are pretty short, the wall is actually forced to jump, because the kicker will try to use opponent's disadvantage to score, in this case a fly over the wall free kick. If the defenders are tall you can let them stand still, unless the kicker's ability is very good.