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L74A2 to say that you should abide by a reasonable request to show them.
I don't think, as others have suggested, 74A2 says that at all. It infers that to reply to the request "*!??? off" might be a breach however "I'd rather not" is not offensive and if the other side wanted to argue it spoilt their enjoyment I would be happy to argue that their request spoilt mine. I have seen, on one occasion, a request turned down, opponent reach for the cards anyway and a large clenched fist descend to the table about 1mm from the encroaching hand(missing was possibly accidental). That, of course, was before the days of zero tolerance.
Now don't get me wrong. I have (usually) no problem with opponents seeing my cards at the end of the hand and would only object if they took loads of time with this, we were late starting the round or they were rude in their request.
One reason, especially in a tight last set to see cards is to improve the accuracy of estimation of likely gain or loss. I guess that if I felt the opponents were likely to refuse I might play my contract out to trick 13 so as to ensure seeing the cards! I await someone telling me I am not allowed to.