I was always told that when taxiing one should observe the vicinity of objects and decide how fast to taxi depending on how likely you are to hit them..
i.e. On a marked taxiway or runway, taxi speeds should be quicker. Around Aprons and parking areas with other aircraft, cargo bins, trolleys, air bridges and structures. The taxi speed should be slower as these objects may leap out and hit you when you least expect it.
I guess this is one of those airmanship things that is slowly being lost..
