DS is a Firstie this year and I can share how this turned out for him 3.5 years ago when he started. Received 5's on AP US History and AP European History and was able to validate out of Plebe US History and World History, but in it's place he took the senior level class Art of War a full year 2 semester class every Firstie has to take that encompasses history, political science... an in depth understanding of major wars. Even if you validate out of class, your schedule must still be filled up with similar classes and in this case as a plebe he is thrown in with Firsties to take Art of War. Did fine with the class and now in his Firstie year it has opened up two spots in his schedule that quickly got filled up with a capstone project and research project with a professor. He was rewarded for those AP classes but it was 4 years later when he got to choose how to fill up the empty slots.
For what its worth, here is the other side. He took AP Calc and received an A but chose not to sit for the AP test. He wanted to take calculus over again at WP so he could learn it their way. He figured without the AP test score he would automatically get slotted for 1st semester calculus. During plebe testing at Beast he tested high enough to validate out of calculus and move into 2nd semester calculus. Since he didn't have the outside test score to lend credibility to the GPA or the plebe test, he had to take the plebe test a 2nd time to prove he had mastered first semester calculus. The best laid plan backfired on him and trying to game the system doesn't always work. WP will always push you to go further than you think you can.
These are very specific scenarios and may not be the norm, but I have read enough to know this is a good indication of how the system works. Best of luck with your decision.