definitely think ab work is necessary... even for people who don't need it, they should do it anyways just as insurance. I've had loads of lower back problems that put me out of commission for awhile, all because I skimped on ab work.
I'm not a fan of any type of sit-up... maybe the McGill Crunch where you lower back is supported. Hanging leg raises are good only if you do them right, if you don't know how to do them properly, then go to a reverse crunch instead on the floor. I also like side planks, front planks (on elbows and in push-up position, with weight added eventually), ring fallouts, and 'stir the pot' on the swiss ball.
edit: I personally wouldn't do any type of twisting motion at the lumbar spine, that's also fucked up my back. Work on anti-rotation instead (i.e. pallof press). Before jumping into rollouts/fallouts, I would do 3-4 weeks of planks first (increasing duration each week, i.e. 3 x 60s, then eventually adding weight).