Actually negative energy is required for a spacelike wormholes stability. Not for its existence. Even if such a wormhole were stable it still would not be transversable because all paths connecting these particular external regions without hitting a physical singularity would also be spacelike. Even though wormholes are currently used to refer to the spacelike hypersurfaces connecting different external regions, I like to think of geodesics connecting other various external regions as timelike wormholes. There is no question of stability as these are not hypersurfaces. They are geodesics. These geodesics are natural paths and so nothing prevents them from being transversed. For a more detailed explanation of what I'm talking about see-
http://home.aol.com/zcphysicsms/wormholes.htm