When to signal, or not to...

[Uncle George]

For me:

Obvious foul ball, a verbal "foul" (however I wait until it comes to a dead stop, hits fence, goes over fence)

Called strike: Simple "strike" voiced followed by proper arm mechanic
Swinging strike: Proper arm mechanic
Foul tip: Proper foul tip signal followed by proper arm mechanic
Check swing when batter goes: Point to the batter with verbal "yes he did" followed by proper arm mechanic
Bunt attempt strike: Point to batter with verbal "he attempted" followed by proper arm mechanic.
Usually don't do the "pull the string thing". Here's my reason. I don't keep track of big situations as fall as when is a good time to "pull the string" thing.
When on the plate and afly ball is my responsibility, a simple visual "out" or "safe" call", whatever the play calls for. I also verbal it loud enough for my partner to hear it.

When in the field, on a close play I may voice the call a little louder.

On a pick-off, if there's a tag, a simple safe/out, depending on how close the play is.

If there is no tag or tag attempt by the fielder, I don't make a call/sign.

On an obvious play by the defense, a simple out/safe, again depending on the closeness of the play.

Fly balls (my responsibility), a simple visual out call or safe call on a no catch/trap of ball.

I was once told by a veteran ump who has done several HS state tournament championship games and college ball as well, signal (either verbal or physical) something on every call.

George