I agree with what Doc E says here about not having to say whoa,
but for me it is a way of shuring things up. Mine was doing pretty
much what Doc described and then decided to go a step further.
This caused some problems for me as the dog caught 4 wild birds
that year and I had to start voicing the whoa comands. So in some
cases whoa has to be used to prevent further headaches later. For
me it works good.