I find it way easier to leave the background of the scene blank and drop the map in as a tile. You can then drag and drop to resize it to the grid. Just remeber to hold SHIFT to disable grid snapping.
This is something I have never even thought to do before but wow, what a simple and effective solution! I'll edit my comment with this suggestion because it probably works even better this way. (Just make sure to send that tile to the back layer before adding any new ones, I guess.)
