on large taildraggers u MUST lock your tailwheel to improve your direction stability, otherwise you'll go trimming grass on the sides of the runway. There are different kinds of tailwheel: the Mustang has a steerable mechanism that was activated when u pushed the stick forward (so that you could actually use it only during taxiing), otherwise it would stay in semi-locked position (with only 6 degrees of movement on each side); the Corsair doesn't have a steerable tailwheel, but it's lockable as well.
The rule is that your tailwheel must be locked at all times apart from when you need to steer on the ground during taxiing.