Although a tight fit may cost less and sound acceptable, you should consider how much extra space you''ll need to maneuver in and out of the slip (especially in cross winds and currents) so that you don''t hit piers, or adjacent boats. :^(
Also, once tied-up, your boat should have enough additional room to move in the slip so that it doesn''t hit anything during tidal fluctuations and wind and current movements.
However, if you know how to line-up your boat ''perfectly'' (regardless of wind and/or current direction and speed), and there isn''t much of a tidal fluctuation, then get the cheapest/tightest slip available. YMMV.
~ Happy sails to you ~ _/) ~