I just added and submitted to the store the FlashMe app with telemetry built into it. Its nerve racking (this is the first time I am going to actually see how people use it!!!).
When inserting the telemetry points, I struggled over whether I should make it opt-in or opt-out. I really wanted the data and I wanted to be able to really understand how people are using the app so that I can improve it. To achieve this, it was really tempting to make the app opt-out, so that by default I would be getting telemetry from people.
At the end of the day though, I decided to make it opt-in. The decision became simple when I thought "As a user of FlashMe, how would I want to be treated?". Currently, FlashMe has no feature that requires internet use. If I made it opt-out, I would be using my users internet connection purely for the benefit of the FlashMe app.
Maybe in the future, when I have some service running online and my users are getting some benefits from being online, then I will reconsider this stance. But one thing is for sure, more valuable than any data I can get from telemetry is the good-will of my users. And that is something I never want to compromise.