I called my 20th National for PONY last year in Harlingen, TX. There is no rule against cheering. Now, of course, if they yell or scream just as the pitcher releases the ball and you think it is obvious enough, you can warn them.
I'm good at the rules and if you have a question about this, contact Dan Vaughan. He is over all the Softball in the South and Southwestern United States. He is also the tournament director for the Nationals down here.
d.vaughan@pony.org
He will usually reply on the weekends.
Rick
