If you intend to use SSH protocols to access your phone, then yes, you really must change the default password (both root and mobile) to something other than the default of "alpine." So, if you install the Cydia tweak OpenSSH (for example) - change the password(s)!
That's really all you need to do as a jail broken iPhone is as secure as a stock one in all other aspects. And, if you don't use SSH (or install any SSH tools, you really don't need to change the passwords.
Marilyn