Please know I am deliberately playing the Devil's Advocate. Read below.
Having read and responded to many questions. I've learned that many take the position that crunches are a no no and many have referenced researchers such as Stuart McGill to support their position. Here's the scenario:
What exercises or movements would you use with a client who presented with an increased lumbar curve? Let us all assume that this is a healthy client, all assessments have been done and he/she doesn't have thoracic kyphosis or osteoporosis. Is a crunch also known as spinal flexion a no no? Too, in your experience as a personal trainer, do you encounter more people with excessive anterior pelvic tilt or posterior tilt? As you have been doing, please use science to support your position.