This would be like a 6-blocks-back block chain lock-in. Maybe not a bad idea at this point (unlike the early days when legitimate users might throw lots of new hashing power at the network or there might have been network splits because there were so few nodes).
I think there would have to be an override, in case there WAS an hour or two or three network split.
Or maybe some attempt to detect double-spends (e.g. more-than-5-block-split OK if all transactions are valid on both chains).