If the turn direction is not specified, the pilot is expected to turn the shortest amount of degrees. Actually I don't know if this is mandatory, but for sure an ATC expects the pilot to do that and pilots try to comply with it.
As you can imagine during a holding it can be chosen to continue the turn for 200 degress rather than going back and change the direction to make 160 degrees. Probably it's a matter of shortest time and workload rather than lower amount of degrees...
I think what I wrote can be considered the "accepted rule", is there a written one?