I think it all depends on both the supervisor and the employee and how they work together. It must be both, or you run into issues. Because we have such a great boss, I am sure he gets about 5-10 free hours of work from each of us each week. (counting weekend work, working late, and missed lunches).
In our office, we have no clockwatchers. The only one we have has two young children who must be picked up at a certain time, so that is not clock watching, that is survival. He can stay late anytime he has 24 hours notice (so he and his wife can take the kids to the GPs). As long as we are working, our boss approves comp time as we need it. Also, our boss is not a clock watcher. If we come in a few minutes late, then he knows he will get it out of us later in the week. We are treated like adults, and everyone in our office acts like adults. I know of some offices who have employees who "run out" real quick for a Drs appt or dentist appt, and don't put in a leave slip. That is fine for them, but because our boss is so great about NOT clock watching, we all put in a leave slip. In fact, I needed to run to the store during lunch and came back 30 minutes late due to a traffic jam, and I put in an hour leave slip (we can't put in for less). My boss said I did not have to, but I told him he treats us like adults and gives us repsect like adults, so the least I can do is do the same.
We all work late, and many times we do not put in comp time slips. It depends on the day, if I know I "messed" around more than what I should have, or if I did not accomplish as much as what I should have, then I will work a little late and not put in a comp time sheet. I have been known to bring work home and not put in a comp time sheet. Again, my boss is a great person to work for. He wants to see results, and as long as we are giving him results, then he treats us like adults. SO, we keep giving him results, which in turn means I can take a longer shower or do another load of dishes and not worry about him watching the clock, and he knows we don't either.