Parking fees downtown seem to come and go. MSS and El COrtez have never charged anything as long as I've gone to Vegas - but both places require a longer walk to the Fremont St canopy. 4-Queens always has free parking validation for slot-club-members...simply get your parking ticket validated at the casino cashier.
Places that I sometimes park free and sometimes can not: Vegas Club, Binions, Fremont, Fitzgeralds