I never did before, because I always read that low body fat levels will show the abs, and training them will make the waist thicker, and is a waste of time, bla, bla, bla, but I've never had them.
Last year before my first show, I trained them almost every workout, and guess what? They appeared like magic! What do you know? So ab work is necessary for them to develop and show.
The best exercises IMO are hanging leg raises. I do those at the end of my workout. I also do 4 giant sets as follows. The Nautilus crunch machine, seated machine sit up, knee raises then hyper extensions, so a total of 16 sets. I do that at the begining of my workout precontest, then after I train, I do 4 sets of hang leg raises.
Last year was the first that I had any ab development at all.