Safe to run cron jobs using wget and user with full admin rights?
It's better to limit the cron user to the minimum permissions - but I'm not 100% sure what those permissions are.  E.g. if you don't give access to CiviEvent, can you use smart groups based on participant status?

The reason why it's unsafe to give a full admin user cron access is because you're leaving an API key (essentially a password) in plaintext on a server.  Depending on how wget is configured (with HTTP GET instead of POST) you're also leaving the api key in the server logs.  Someone who accesses one of those can escalate their permissions.  This is especially true if you're using wget from a remote server.