I fear that I have been guilty of putting my legs up on an empty chair. During law school, I had a tendency to choose my seat such that there was as often as possible an empty seat to my left, and I was especially particular about needing space to my left in my evening classes. I used the extra space to spread out my books and notes, but during my last semester I realized that I was also using the empty seat for my left leg. Once I became aware of what I was doing I made a conscious effort to stop it, but what I found was that over the course of an 80 minute class my knee would become increasingly pained, and the only way to relieve the pain was to prop my leg up on the chair next to me. I've had knee issues since law before I started law school, so in retrospect I think I was subconsciously making sure there was always an empty seat to my left not for the extra desk space so much as for the chair to prop my leg on. My knee always hurts more as the day progresses, so that would explain why I was even more focused on procuring an empty seat for my evening classes.
I never put my shoe in the chair, I would sit at a slight angle in my chair, with my left leg diagonally across the corner of the empty chair, so that my knee was on the chair but my calf/ankle/foot were not touching anything. I didn't necessarily think it portrayed a professional image to prop my foot up on a chair, but I couldn't come up with any other solution to relieve the pain in my knee. I suspect that if/when I have to go to court I will take painkillers, but I am hoping that I will not have to go to court everyday as I did to class.