If you wish to minimize distortion tossing water or a wet towel on it is the last thing you would want to do. It would be better to pre/post heat your weldment, and allow it to cool slowly. Distortion in weldments is usually due to uneven thermal strains created between the leading/trailing sections of the weld. The trailing section has been melted and shrinks as it re-solidifies, which tends to locally pull the two parts of the weld together. It might also be helpful for minimizing distortion to make the weld by alternately welding small sections from one side to the other.
If the weldment is fairly large, tossing a wet towel on it immediately after welding will probably just result in the towel burning after the water boils off.
