While I sold my jeep before I became a daily kilt wearer, I had worn one more than once with the top down (top came down in April went backup in October). It really isn't a issue. Wind blown in weird patterns in a Jeep, and between the knees it is almost dead calm ever at 60+MPH.

(Note this is in a Wrangler with the 1/2 doors. Milage may vary in a Wrangler with the full doors or in a CJ5/7 with no doors).

Adam