The squid that you get from the supermarket should be smell fishy at all but rather salty. If your squid smells fishy and rotten, chances are it is no longer good and you should throw it away immediately. Just make sure that you take the time to check and see how your squid smells before you eat it, because the last thing you want is to develop problems with food poisoning.
It will also be important to make sure that you properly store your squid so it will keep for as long as possible. You will find that keeping your squid in the freezer is a good effective way to keep it fresh, especially if you are not going to be using it for a while.