The take away i see is that "one should be deterministic about their shot and not be guessing where to hit".
The wardlaw directionals look very basic to me and may help certain players.
There are several different strategies in tennis game and thats why i love it. I play it a little different. To me at a high level there are two deterministic strategies
1) attack opponents weakness
eg:Weak backhand. Apply pressure consistently attacking the opponents backhand and try breaking it down. I take it to extreme sometimes and repeatedly hit to opponents backhand regardless of where they hit to me.
2) Play to your strengths
eg:Strong forehand. Play as many forehands as possible and dictate the play.
As to when to change the direction of the ball, my strategy is simple. Whenever i feel i have an open court or an opponent is out of position I pull the trigger.