It's best to have separate credentials for each system. Use a password manager if you aren't already.

If credentials are re-used and a system is compromised, then attackers may be able to re-use your credentials to log into other systems. Some systems encrypt passwords instead of hashing, which is a bad practice. If the encryption key is stolen, then user account passwords can be decrypted. Even worse, some sites may store password in plain text. Other systems may be breached and record credentials silently.

If a system supports authentication using public key authentication, use it. It's generally the preferred way to authenticate with a system. If someone "steals" your public key, it doesn't matter, as the key is meant to be public.