The exact laws vary from place to place, but they all generally say the same thing: you must have a clear view of the road behind you in order to operate a vehicle safely. This means that you cannot have anything in your car that blocks your rear view mirror, even if it is just a small object.
If you are caught driving with a load that obstructs your rear view mirror, you could be ticketed and fined. In some cases, you may even be impounded.
So, before you hit the road, make sure that you have a clear view of the road behind you. If you have any doubts, it's best to err on the side of caution and remove anything that could potentially block your view.