Gridlines are adjustable in reports by the same means as in forms. Double-click the report selector button in the upper-left hand of the report to open the Report Properties. The Format tab contains the Grid X and Grid Y properties:

 

 

Enter a number from 1 to 64 to divide each square inch of the report into that many increments. If you would rather work without the gridlines, click View à Grid to remove the checkmark and format the report by sight.