If they're late opening, it's not rude at all to knock. But when you get inside, check what the store clocks say. They may be 5 minutes slow, which would explain them opening late on a regular basis. Mention it to the manager if it's a regular thing, and if that doesn't do anything, I would mention it to corporate.
I wonder, though, you say you're seldom in the area. How do you know that this is an ongoing problem? Let's say you're at that store once every two or three months. I'd say a store opening 5 minutes late once every few months isn't a major problem, it's something that happens sometimes.
Also, do you wait outside the door that long every time you're in the area? I know that if I was on the floor of a store getting ready to open, I'd be less likely to open on time if I was alone on the floor because the manager was still counting out the drawers or in the back if I had seen someone standing outside for at least 15 minutes before the store was due to open. I'd risk making the customers wait five minutes versus having someone else on the floor just in case something were to happen.