When does the U blow his whistle?

[jTheUmp]

I've been U for 4 years now... you're pretty much got it right.
The other one I would add: after the play ends when you've got a foul to report (to let your WH and the rest of the crew know).

Spit our your whistle at the snap, and from then on use your voice primarily. Things like "that's it" or "he's down" or "it's over" can do a great deal to avoid any late/unnecessary hits, and lets players know that you're there and that you're watching them.

I rarely blow my whistle more then a few times a game.