The reason why it is important not to mention it if you had been living together for more than a year when you got your PR is because if you had, she would have qualified as your common law wife and should have been included on your PR application.
Yes, I guess I should have been more specific in my warning. If you had lived with her for a year before you got your PR, you should have declared her on your application, and declared her as your common-law partner when you landed. If you said you were single when you landed, when actually you had already been living with her for a year, you won't be able to sponsor her later.
So,
if this is your situation, your best bet is to hide the fact that you had been living together for a year or more before getting your PR.