i go all the way to the top, that way i have a week between waterchanges.
one of my tanks is sort of off a bit and that one i fill it til the water is at the edge of the low corner. usually this allows the air bubbles to clean the algae off the glass.
1 do have 1 or 2 cheaper 10g's that will leak if you do this, but all my all-glass tanks are fine with it.