Above all else, you need to not surprise users. Most auto-follows after an app authorization surprises users. Even if you warn them ahead of time.
To best mitigate this, make it abundantly clear if you’ll be doing this. I recommend a large checkbox, defaulted to UNCHECKED, that says “[ ] and follow @username” – if a user starts the authorization process with you and this box is unchecked, don’t follow the @username on their behalf. If it is checked, go ahead and do so.
If it’s important for your application that a follow relationship is established, I would make sure that the advantage and disadvantages of following or not following the account in question are clearly communicated. Perhaps after the user has authorized your app, you could then again strongly suggest the follow action to the end-user, should they not have opted-in the first time around.
Defaulting to an opt-out scenario causes more user confusion than it’s probably worth.
