I certainly prefer to signal the count (and when coaching to have it signaled) every pitch. It doesn't slow anything down, hurts no one, and brings any error to light more quickly. Just don't see a reason not to. Even if it isn't specifically necessary on 2-1 what does it hurt? Maybe not verbally until the next pitch could result in a walk or strikeout. I would say only about 25% of our umps signal the count every pitch though.