Be careful about when you interrupt your users

It's common to want to ask your users something. Maybe you want to prompt them to rate your app, provide feedback or update to the latest version.
The way I see most people do this is with a MessageBox being displayed when the app is launched.

I don't think this is the best approach.

I hope you're creating apps with the user and what's best for them in mind.

When a person starts your app they probably have a task, goal or purpose in mind. People don't launch apps to rate them or send feedback, although the desire to do this may come up while they're using the app.

By prompting the user to take some action that you want, you're putting a barrier between them and what they really wanted to do when they launched the app.

Adding a minor frustration just before you ask for a review may not be the smartest thing to do.

I suspect that a better approach would be to prompt the user with such requests when they've done what they launched the app to do.
For some apps it may be harder to identify just when the user has completed what they wanted to do.
As a general rule, prompting when a person leaves the app, rather than when they start it seems to make more sense to me.

Thoughts?
I'd love to hear from anyone who's experimented (and tested) in this area.