You have to be careful.
I know I've ruined CD's just being read from in a drive from bumping a comp that was in a vertical (and horizontal) position. Too much of that probably will throw the laser calibration off... and I'd imagine even more serious damage when the drive is actually burning.
Not saying you don't have it in a safe spot, but the fact that it's vertical has to make the laser work slightly harder. I don't know for sure though.
You'd probably be better off asking a question like this at the cdfreaks forums.
