Because most deck frames are not perfectly level you may need to make some adjustments to create the appearance of an even deck surface. Because the size of joists can vary from board to board you may want to install wood shims to raise the position of decking over low joists. High joists may require you to plane down the surface height of the joist to accommodate a level floor height. As the decking ties the frame together it should help correct most variations from joist to joist. The weight of the frame and the forces of gravity should correct most minor deviations.