From what I have read, an attack to the network does not need to be 51%. The likely hood to be succesfull with above 51% is higher. So it really depends on how you want to look at a secure network.
Although it is called a 51% attack it is really more a >50% attack. As long as the spawning genesis has more hashes and can be extended out to form a chain (or subchain) that can be accepted by the parent, then a 51% double-spend type of attack can occur.
Fundamentally speaking, any coin that has less than 50% of all of the possible hash for a given algorithm shoulbe be considered unsecure.