OK, Arcana, here is what I do (I used this when I did the Thunderspire maps and what not).

Instead of using the Stone Grey Tile bitmap fill use the Stone Grey Flat bitmap fill.

Then make a GRID sheet or simply use the Default GRID SHEET, and in that sheet use the following effect:

Bevel: Length 3; Strength 30%; Invert Colors 1; Fade 0

This will make it appear as if the floor is tiled wherever you draw some lines. (every 5 feet, every 10 feet, etc)