We have mirror, and pattern creation and i can't believe that a grid overlay isn't already in the app. :) It would be hugely useful. At LEAST a cross that shows the middle of the tile horizontaly and vertically or any simple "mark"... A ruller, anything is better than nothing. :) It would be handy while planning patterns and stuff like that.
The way i "do" it now is pretty rough, i just overlay a tile Generator + Level invert on my flow, but this is processed along the other nodes so it's not even close to a usable way. :)
Maybe it should be added to the SPainter too.