If you can work your abs out everyday and them not be sore, you're not working your core hard enough. I do abs every other, or 3rd day. That seems to work best for me. As for being able to work them out everyday, I wouldn't think you could because as you said, they need to rest to recover and grow. I have heard personal trainers at a few gyms talk about how your abs recover faster than the rest of your body. But, I don't buy that. My core feels like it's blown out after my core workouts, there is no way I would be able to do it two days in a row, let alone everyday!