One nice thing about the Steam client software (and store website) is that it uses two factor authentication. The first time you log in on an unfamiliar machine, you have to input a code which is emailed to you.
Even if your Steam client and forum passwords were the same, your client account still secure as long as your email password is different.
Even if your Steam client and forum passwords were the same, your client account still secure as long as your email password is different.