There is nothing to stop someone forging a paper form given in person, the same concerns apply.
We have received emailed proxy forms from owners – if there is no reason to query their validity that is fine, but if for example it came from an unknown/strange/odd email address, or there was some other reason to question its validity, then you might make further enquiries. But as I said, someone could rock up to the meeting with a paper form that had been forged just as easily.