I doubt that will be a problem. Some people think that if a muscle is much stronger than it's opposing muscle (the muscle that reverses the movement) it will cause posture problems. As far as I know there is no direct evidence that this is a case and the posture problems are probably just a result of muscle tightness that can be fixed with stretching (granted building muscle strength leads to muscle tightness which might be why some people come to the conclusion muscle imbalance causes the poor posture). Maybe someone who has looked into this more will reply, although I've never heard anyone being worried about ab overuse before.
If you do want to build up the opposing muscle group it is the lower back that reverses the crush movement of the abs. Exercises are hyperextensions, good mornings and stiff-legged deadlifts. You can do hyperextensions without weights if you don't have any. Search google for descriptions, or try
www.exrx.net. I suppose you can stretch the abs at the top of the movement of hyperextensions.
Besides possibly lower back muscles how strong or tight your abs are relative to other muscles is unimportant.
Personally I'd just keep doing what you are doing. Maybe try hyperextensions and do the stretch at the top of the movement (just pull back to stretch the abs - that should work, I'm talking theoretically, I haven't tried it) if you don't mind the exercise. The only thing that could possibly be affected is your posture as far as I know, so I wouldn't worry unless you start to notice your posture suffering (and the only thing that might happen is you will find it harder to keep your back straight).