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There might be some very specific cases where certain types of traffic do not work without special configuration - for example the router will not process SSH traffic until you have generated RSA keys.
Just to clarify this, this would be SSH traffic to/from the router as a host, SSH traffic would normally transit a router like any other traffic or as Rick described, i.e.:
In general terms we would say that yes the default is that a Cisco router with no access lists will allow all traffic to go through (assuming that it is correctly configured with IP addresses etc). But i believe that your question is intended to address the general behavior and so I would say that the answer is yes it allows all traffic through.
