Dave,
My head and neck cancer team was emphatically against the notion of "holding something in reserve", for the reasons Brian stated so eloquently above. While I wasn't happy to hear their advice at the time (when I thought my surgery should be enough), I followed their recommendations for radiation as soon as possible after the surgery and have never had reason to regret it. (That was more than 17 years ago.)
I was a 39 year old non-smoker when I was diagnosed, so I know it can be pretty daunting to hear, at a fairly young age, that you have only one shot at radiation. While I had some of the typical side effects for quite awhile after, I'm thankful that I was able to heal to the point where my quality of life issues became minimal. If you have experienced head and neck cancer professionals recommending that you have radiation sooner rather than later, please pay attention to them.
Cathy