No. Always allow the user to opt-in.
You could prompt the user the first time the app is ran, with a message explaining why it would be a great idea for them to join, but I would default off.
Automatic opt in causes the PERCEPTION of your app to be "shady" for some people. This perception has a big influence on the total UX.
And what's more - this can, and will, extend to other apps you write - potentially loosing other future sales.
Make your app above-board, and you'll have more people using your app - even if fewer people opt in, you'll get more data.