Yeah, I didn't think that was entirely correct. The discussion I had with CIC was, "How soon is too soon to renew your PR card?" The representative said there was no legal obligation to renew your card; you only need it if you are travelling. I asked about returning from the U.S. by car and the that's when the rep said you don't need to show your PR card when returning to Canada by land, any form of official ID that shows you live in Canada is acceptable.
BTW, the answer to the original question is that you can apply to renew your PR card whenever you want.