I think it depends on the look you're going for.
If you like the blocky, built up mid section that some of the fitness models end up with, then I would say you need to work your abs every day.
If you desire a smaller, curvier look... more of an indent between your hips and upper body ... cut back on the ab work.
This is how it works for me at least. I recently proved this to myself while healing from a broken wrist. For at least 3 weeks I couldn't do any ab work because I was casted almost to my shoulder. During those 3 weeks, I lost an inch in my waist. I kept the definition, but lost that bulkiness that had palgued me for a long time. I hadn't realized that the reason I was looking that way was because I worked my abs so much. The blockier my mid section got, the harder I worked... thinking that's what I needed to do to trim it down.
Everybody is different, but for me it was quite a shock to realize that I like the way I look better with less ab work! I went from every day doing ab work, to about 3 days a week.