Roof bars and mounts are the most useful, IME. Any rear mounted rack gives you the issue of rear access and is more likely to result in bodywork damage. Of course if you have roof mounted bikes, why not hang something from the mirror to remind you? Otherwise going through height restrictions can lead to excessive amusement AND eencie weencie bodywork damage. PLEASE DNAMHIKT. Rear mounts are best suited for the odd convenience trips (I have and use both types)
Oh yes, and there was the time that a (1/3) bike flew off the back carrier at 70 but fortuitously the magic bungee kept hold and the only damage was to the end of the SPD pedal as it was dragged along, and a small bit to the rear rack.