The general rule of thumb would be: the stiffer the better when it comes to racing.
Take note though that there are a (very) few select cars that will handle a bit worst with the chassis reinforcements. In GT5, at least. The only one I can think of right now is one of the premium model Ferraris. I would wait til a reputable tuner can chime in with some more concrete info.
Yeah i was assuming it would make it better on the game. But like in real life I would think adding more cross chassis stiffness would make a race car worse. Just like i never add horsepower to a race car. It's designed to be that way for a certain reason.
Chassis stiffness is always a good thing; the only reason to avoid it in the real world is added weight... But it doesn't add weight in GT5.
That said, a car with chassis stiffening will require a somewhat different setup from one that doesn't have it.