We clean our glass every couple of days with a damp sponge dipped in the ash and then wipe it with a piece of paper towel. It has never needed anything stronger as the burnt on bits don't get chance to build up.
I am a clean-glass freak though(!), as I like to see the full effect of the flames and it is an almost entirely glass-door modern stove.
Our old ancient wood-burning contraption, which has now been sent to the metal recycling merchant in the sky, used to need Mr Muscle oven cleaner on the glass every single day, otherwise you couldn't even see if it was alight or not.