There are very few companies, in my experience, that are OK with you charging less than 40 hours, regardless of circumstances. If you want to leave early, you burn your leave time.
They are all happy (and in some it is implicitly mandatory) for you to work greater than 40 hours.
My experience is that I just do something reasonable and I've never had any problems. If it's just an hour or so early I'm leaving on Friday I don't even bother to announce it. Sometimes I'll just state I had to deal with some issue in the middle of the night so I'm taking off early tomorrow to catch up on rest or whatever.
Oh, and unless I'm taking an entire day off I've never used any leave time for something like a doctor's appointment at any employer. If I miss a few hours of work one day I might make a special effort to make it up or might not, depending on what's going on in the department and my perceived need of whether or not it's valuable to the team/project. I'm sure soon enough I'll get hit with some overtime for a big project or a late night support call anyways.
FWIW I acknowledge that overtime is part of the deal on occasion working in IT, but I'm not the type to grind away 50 hours every week forever. I might be able to sustain that for a week or two in an emergency, but it's not worth it. I talk about that part of the work culture during the interview process. I would avoid grind shops.
I have not had that experience in reasonable environments. There ARE bosses that count butt in chair hours, but I choose to not work for them. They likely now have an entire team of people that clock exactly 40 hours, after all of the talent left.
They are all happy (and in some it is implicitly mandatory) for you to work greater than 40 hours.