Depending on which company you buy from, the core clock can change, though there is a minimum for it to be able to be called a GT so there is performance guarantee.
For instance I know a store which sells a 6600 GT clocked at 400 for one, but another brand will sell it clocked at 500. Still yet you can buy some which are overclocked and are at 550.
So yes it is true, though you should look around for what kind you are buying and see which gives the best deal of course