We have been advocating clients to not do crunches and sit-ups for years. They are ineffective, can lead to back pain and injury and are just bad overall. There are many more effective ways to condition the core - safely! Planks are a great example. When done correctly they can not only strengthen the core, but they can improve posture and help with lordosis.
Check out this article below for the scoop on why the US Military is getting rid of the crunch and sit-up.
http://blog.bonati.com/us-army-phasing-out-sit-ups-due-to-spine-injuries/