Is it important to work abs? Absolutely (see what I did there?).
But in terms of helping to contribute to a healthy back, they're no more or less important than any other muscle group that attaches to the hip and pelvic girdle. Abs, hip flexors, spinal erectors, glutes, deeper hip muscles like the 'deep 6', even the diaphragm, those all play a huge role in spinal position and function.
The "best" way to work the abs depends on the issues of the respective individual and what they can safely do without compensating out of the movement. Full situps might be great for one person, damaging for another. Planks, same thing.
I'm a big fan of rebounding as a form of exercise. I use one called the Cellerciser. Spend two minutes bouncing on it while sitting on it with feet lifted off the ground and you get an amazing ab workout.
Volkl Organix V1 Pro
Pacific Gut 17 gauge mains at 54lbs/MSV Co-Focus 17L gauge crosses at 50lbs