downhill for sure. I am tall, and lean forward into the guitar from the hips. i also feel like having the higher part of the footstool at your heel raises your leg just a bit, allowing you to use a lower footstool
Uphill (heel lower than the toe) I have a fancy looking flat one that I used for a 3 hour gig the other day and I felt like I needed to get knee surgery after it was over. I an not sure that it was caused by the footstool, but I am making it the patsy anyways.