While a grid is used for scale, its primary purpose is for the placement of miniature figures, 1 square inch being the most common (or as mentioned above 250 mm is another), so creating grids smaller than that offers no benefit. The scale should match the base of the figures - so .25 inches is of no use for encounter scale maps. Also Sapiento, I use PosterRazor a free program that automatically slices large PDF files (maps) into printable chunks able to be printed from the home printer. You can set the size of unprinted area - I usually set to .75 inches to accommodate the least capable printer, its easiest to use that for converting your map to printable sizes in an easy to do method.

GP