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.

The github bit doesn't make any sense. When you leave you just get kicked from the org...

I also separate Github personally and professionally as a FTE. In most countries the company where you work has full access to your work computer, which implies also to your personal github and everything related to it.

As a freelancer they don't have access to your computer so things are different.

That's what the OP thought about Trello and look how it ended...