[MichaelGagnon]

Depends on your definition of ethics. * Consequentialism ethics: make sure the benefits to users outweighs the harm to them (in your judgment) * Deontological ethics: do the morally right thing (for some definition of morally right) * Or pick some other ethical theory

Pragmatically, you want users and prospective users to feel that you are being ethical (assuming they know what your app is up to). Technical people often have a hard time putting themselves in the user's shoes (when users are non-technical). Just pitch your idea to some non-technical friends and ask them if your plan sounds "ethical."