I am also a reversed footstool player. I read about it almost a year ago, I tried it and it really helped me! The pain or stiffness in my lower back disappeared from that very moment.
The explanation is simple. The non-reversed footstool stretches your calf muscles, they again pull at your hamstrings and this effects your lower back muscles.
Of course this is not a general advice. If you don't experience any problems by now, just keep doing what you do. On the other hand, if you experience some problems, why don't you just give it a try!
I am not a physiologist, but as a former long distance runner I experienced more than once how local problems in your body can be solved by changes at another location in your body. A simple pad of 1mm high under the sole of my friend's right shoe, solved serious back problems (even walking was painfull).