It's generally accepted that you should whisper during:
- Church, especially if you're in a pew.
- A library, because everyone else is trying to read.
- A courtroom, unless you're a lawyer.
However, you should not whisper during:
- A rock concert, because the music is too loud.
- A horror movie, because the jump scares are more fun.
When to Whisper in 3D for more information.
Whispering in Public Places is a whole different story.