I haven't yet done it -- but I'm told over and over that enclosing the front wheel as much as possible is good. There's air being moved around by the rotating wheel, and since some of that air is being forced in the direction you're traveling (forwards) you've guaranteed yourself some excess drag and turbulence by having that dirty air there. Cover the wheel as much as the rules will allow.
Then there's the difficulty that was presented by the rules -- stating that no streamlining is allowed in front of the front axle; and that the front fender may cover the top half of the front wheel (I generalize -- the rules are a little more specific). The difficulty is/was that the front fender covering the wheel is de facto streamlining, but streamlining de jure describes the bodywork, nose piece, etc, and does not include the fender. There was a whole heck of a lot of discussion on that -- and maybe a rule change, too, to clear up the cornfusion.
Anyway -- cover that front wheel!
