I'm no saying your wrong, but you must take into account that a chain puts alot more pressure and tension, side to side, than your hand can ever exert. If the lock ring is just snug and the cassette does not move with a light tug of your hand that does not mean that the chain with not move the cogs on your hub body and tear it apart. It's better to tighten it slightly more and be sure that the cassette will not move at all.