My position is that the assumption that C14 levels have been fairly constant is a reasonable starting base for dating items.  However, this assumption is not necessary because of the existance of calibration curves.  Basically, we get almost the same namers from calibration curves that we get from just assuming that C14 levels have been constant for the last 10,000 years.  We have two independent ways of getting at the same data.  The chances of both methods giving us mostly the same data by pure chance is really small.      