I tried that, but I found after a while that it gets somewhat tiring (and makes my wrist sore). You have all that unsupported weight at the top of the Kindle producing torque.
But then again, I presumably have bigger hands/longer fingers and have to hold it closer to the bottom to get my thumb in that position.
What I do is sort of cradle it in my left hand, my thumb up at the top of the screen, and then use my pinky (which curls under over to the right side) to press the button. Which works, but isn't that comfortable and has a tendency to twist the Kindle. So mostly I use my right hand to press the button.