Please see my comment. Trello uses HTTPS which means all your communications are encrypted, but the data for Trello is not encrypted on disk. We might offer this in the future for pay.
Some people (me) are comfortable storing usernames, account numbers, and passwords (or at least password hints) to trivial things like frequent flier accounts in cloud-based services (e.g., Google Docs). I do this regardless of whether Google encrypts the data on disk because the consequences of failure are pretty low.
In general, though, there are much better resources out there for password management, and going with something that is specifically designed for storing that kind of information might be a better bet.
In the end, no one can answer this question for you, except you. I've nominated this question for closing because "good enough" security questions tend toward arguments between people who have different ideas about security, with no clear answer possible.