I do at the end of every workout (3-4x a week), but it's not ab work in the traditional sense. I barely ever do crunches. It's usually what most would consider "functional" work. For example, farmers walks, planks, pallof presses, ab wheel, overhead pallof presses, suitcase deadlifts, etc. I used to never do any ab work except the occasional crunches, but after a back injury it made me reevaluate my training completely, and core training (along with flexibility, mobility, and soft tissue work) was one of those areas.
For a very good article on core training and along the lines of what I do, check out this series by Mike Robertson.
Understanding your Abs Part 1
Understanding your Abs Part 2