Is there any merit to this?
Possibly. Think of it like parking on a hill and leaving your car in gear/steering into the kerb, in case the handbrake fails.... it's a precaution, rather than a requirement.
In the case of a Cessna, the fuel tanks in the wings are very long, wide and flat - so it wouldn't take an excessive amount of bank to allow fuel to slowly move from one tank to the other. If that happened to a great enough degree, in theory the fuel could adversely affect handling.
Chances are that the ramp isn't off-level enough to require it, but assuming that it does block the valve (I'd assume it does, as I'm not sure how else it would work) then there's little harm to be done, and some potential benefit in some circumstances.
Does it actually block the crossflow valve?
No idea, I don't own a C172: it seems like a logical conclusion, but I'd check your manual or with the manufacturer.
Just remember to turn it back to dual feed when you set off.