unless the indoor storage is extremely humid, like may be found on a coastal area, i'm almost certain all you will find is a light coating of very fine surface rust.
as others have said, a few slow stops will have the rotors back in shape.
as to having rotors on the shelf, i have a couple of sets of those for my caravan.
as i turn my own rotors on a real lathe, i came to discover if one makes an arbor that the rotor bolts to as when it is in use, it only takes a couple of thousandths of a cut to true it up.
i sand blast them first and paint the hub, the back side, and between the fins [just hose the devil in there !] with black epoxy appliance paint.
i realize most can't do this. just commenting on what works for me.
your mileage will vary.