If you have no plans of going outside Canada, don't renew your visa, it's a waste of money since you wont be needing it anytime soon. For the extension, it is up to your discretion and situation. Some people do apply for an extension to be on the safe side, some simply wait for the OP. In my case, I had to apply for an extension 3 mos before my actual permit expires because I'm taking care of an elderly with a critical condition, I had to make sure I'd be able to stay in Canada in case something happens to him.