If we don't arrive til 9 pm do we need to call to let you know?

U should always call

It depends on if you have already paid for the room or not. if you have paid already then they're not worried about the room or what time you get there. If you just made the reservation with your credit card but haven't paid for the room then you must call them and let them know that you are still coming so they know not to get rid of the room to another guest.

No

I arrived at midnight and didn't have an issue. We called and let them know

Yes

Probably best but i arrived after 9 and did not have a problem