Everybody must preserve what was defined by common ecumenical decisions, but a particular opinion of a church father or a definition issued by a local council can be followed by some and ignored by others. Thus, some people customarily shave their beards; others reject this practice by local conciliar decrees. Thus, as far as we are concerned, we consider it reprehensible to fast on Saturdays, except once a year (on Holy Saturday), while others fast on other Saturdays as well. Thus, tradition avoids disputes by making practice prevail over the rule.
In Rome, there are no priests legitimately married, while our tradition permits men, once married, to be elevated to the priesthood. When the faith remains inviolate, common and catholic decisions are also safe; a sensible man respects the practices and laws of others; he considers it neither wrong to observe them nor illegal to violate them. (To Pope Nicholas I of Rome Ep. 2, PG 102, cols. 604-605D)