It is also important to understand and investigate whether free users are helping your business or not. Not all business have network affect like Dropbox and can benefit from free users.

Sometimes, free users are actually hurting (they will write bad reviews, send hate email and write hate posts saying that "I don't want to pay", etc.)

For example, 37signals Basecamp does not have free tier any more. It would be great to hear from them why they decided to remove it and data behind their decision.