Last week, I overheard two ladies chatting on the subway. They were talking about a manager who didn't allow someone to leave a bit early from work to care for their kid, or something like that. One of the ladies said, "Managers have to do that .... they can't let people get away with anything. Then they will just be taken advantage of."
My first thought was 'Wow, these employees really think that? They don't find it unreasonable a co-worker couldn't leave early?' My second thought was 'Really? Can't managers exercise discretion from time to time? Can't they have some wiggle room to make exceptions without selling the farm? If a co-worker leaves early, do you really feel it's only fair if everyone gets to?'
I think the answer lies in holding people accountable for results. If they don't deliver results then it might mean leaving early is a problem (even 'on time' for that matter). The answer also lies in having flexibility across the board. On occasion this might be to take care of a child, or it might be to beat the traffic out of town for a vacation.
Setting a precedent should be for kindness, respect, flexibility and treating people like mature adults. Setting a precedent for a rule seems like you are focusing on the wrong area. The workplace shouldn't be like a nursery school - everyone shouldn't get the same snack or read the same book.