Open source, closed source, and everything in between.
First and foremost is respecting the user privacy and freedom. This can be achieved with just about every software license under the sun.
Ideally open source is preferable, since the code could be audited, but any iteration of open source that allows you to view the code can achieve the same goal.
Closed source is the least desirable, but if no other options exist, and still respects the user's freedom and privacy, then it is an acceptable option until better one is available.
Just my 2 sats. Other opinions may vary, and I would be happy to read them, but this is my approach at this stage of my life.