Rubber deteriorates with age just as anything else does. No, you don't have to replace them. However, it's convenient to do because you're already at the tire store and, because your tires are already dismounted anyway, they generally don't charge labor to install the valve stems. If you need to replace one after the fact, you'll generally be paying for the part and labor, and possible tire re-balance as well. You'd likely pay 20 bucks to have ONE valve stem replaced after the fact, or you can pay 20 bucks to replace them ALL right now.
That's the gamble you take. Sure, it's a pretty low-stakes gamble. Myself, I'd just have them replaced. For 20 bucks, it's an easy decision to me.