There's always going to be a little bit of fog padding around the edges of the Page if Fog of War is enabled. I'd recommend that if you need to reveal or hide an area that's very close to edge of the Page, zoom in and place your cursor very close to the edge. Draw the FoW reveal/hide rectangle starting there. Roll20 can't register the drawn rectangle if its boundary moves beyond the Page border. Due to this, always draw from the outside and work inward. This method should give you the best results.
