Car rental taxes are supposed to soak visitors (to the implicit benefit of locals), not soak locals. Car share (maybe under, say, 4 hours at time?) rentals shouldn't soak locals who are generally doing a wonderful thing by not taking up all the free parking that is at the center of most neighborhood politics.
So, yes, carve out some sort exception for car sharing. These kinds of taxes are put into place specifically to charge out of towners some extra money (for better or for worse).
