At uncontrolled airfields, I always turn left to vacate. If the taxiway is on the right, I turn through 270 degrees. I'm fairly sure that this is correct.
Not really - Rule 17(7)(C) is the one you're thinking about - however that is only applicable for airfields where take off and landing is not confined to a runway.
For Airfields with a runway it's Rule 17(7)(D) which applies -
"A flying machine after landing shall move clear of the landing area as soon as it is possible to do so unless, in the case of an aerodrome having an air traffic control unit, that unit otherwise authorises."
With that in mind a 90 degree turn is a lot quicker than a 270 degree turn
