[10.2] So where should I put this handler?

Depends on what it's doing and how you want it to behave. The relevant guidelines are succinctly given under the heading "Strategies for placing handlers" in chapter 3 of "Using Lingo" (pp.84-85).

The rule of thumb is, place a handler in a script that's just far enough down the message-passing hierarchy to catch all the messages you want it to catch and none of those you don't. In practice, this usually means defining default catch-all responses in movie scripts, default button behaviours in cast scripts, and frame-specific actions in frame and sprite scripts.

Only a few cases are set in stone: startMovie & stopMovie handlers must be in movie scripts, because when they are executed there is no current frame. Parent scripts should usually be in score or cast scripts to hide their methods to calls that don't use their offspring. And all the generic handlers you want to be able to call from anywhere in your movie need to go in movie scripts.