If you're a freelancer or a contractor, and you're doing work for someone who uses Trello to manage projects, you should sign up with a throwaway e-mail address. That's what I do for Github. That way, if that organization then decides to wipe the account or mess around with its permissions after I've stopped working for them, it's no skin off my back.

Personal stuff is personal, work stuff is work and ne'er shall the twain meet.