I would imagine training all sections would be more effective. The more strain you put on the muscles the more fibers will tear and repair, making them bigger and stronger. Depending on how serious are you, with abs you can do 1 to 3x a week, when I was in my fitness obsession stage I was doing them like 4 or 5 times a week.
Great looking abs are made in the kitchen. For the work out portion, do what you feel is safe for your body.