I have it pretty easy really, it's all about balance. I'm a Quantity Surveyor and my "Contracted Hours", Are 8am till 5pm.
If my projects are all trouble free, it's easy and as long as my work gets done, I'm doing what I'm meant to be, and the company are making money I have a pretty long leash. I can get in later, or chip off for the afternoon etc is not a problem. Book it down as "Site" and have my phone on me if anything happens is the rule.
If I have a BIG problem on a project, and there have been a few!! It's politically sensible to be in a little earlier, or leave a little later and be seen to be making every effort to fix the problem. I might even do a bit at the weekend, if thats what it takes to swing the balance back to Easy street.
It's all about keeping boss happy.