I’m a female, not overweight (no muffintop visible while standing) and I guess I’m a little long in the torso. Most shirt/pant combos leave a little gap. I make sure to cover it at work, often by wearing a leotard thing under my shirt, but I haven’t bothered when I am just out window shopping or whatever on the weekend. (Leotards make going to the bathroom a hassle.) Should I avoid showing a half inch of belly skin? I’m 42 and I look and dress a little young, but I don’t want to look like I’m trying to be sixteen.
I think it depends on where you are. If it’s a business/professional setting, not so good. At a more casual venue, like window shopping as you mention, not terrible.
If you’re sexy no. If you’re flabby yes.
It makes you look less formal. If you’re just out shopping, that’s just fine.
My only concern would be that if there is a little gap when you are standing with your arms down at your sides, there’s going to be a big gap when you reach up or bend over, and that could be a little much for some venues. Also, I’ve found that shirts sometimes ride up when I am sitting certain ways, so again, it could be a problem. It would make me feel awkward and I’d worry about it. But I am also very short waisted: it’s possible this stuff plays different on people with actual torsos.
On the other hand, a shirt that just brushes your pant tops when you are standing arms down but flashes a little skin when you move around is perfectly fine for casual stuff like shopping.
Totally inappropriate for the office - yours or anyone elses.
It’s OK for other areas depending on where/when/what sort of image you’re trying to project.
Personally, if I saw a woman of your age (which is very close to my age, incidentally) walking around with belly showing like that, I would assume she hadn’t yet learned how to do laundry and had shrunk all her tops. Really, I’m tall and long waisted as well. I buy longer tops.
For women of your description, it should be mandatory. Very sexy!
IMHO, it’s only ok if the woman is under 27, in a casual setting, and has a flat stomach.
We’re going to need pictures to make an accurate analysis.
It’s pretty much standard 'round these parts (Budapest, Hungary), except it’s more like one to three inches of bare midriff instead of a half inch. This often includes the business/professional setting and is independent of the tautness of the midriff in question. De gustibus… and all that. Go for it.
If you look like Daisy Duke, Ellie Mae Clampett, or Daisy Mae Yokum, it is a good thing. If you look more like Roseann Barr, not so much.
It’s pretty common where I live; not for office wear, but your everyday going out clothes? Sure. You’re in shape? Show a little skin.
Ha, when I was in Budapest it was that year where all of Europe got insanely hot for, you know, just the weeks we were there. Obviously Hungarians do not know how to dress for a whole eighty degrees, because you totally couldn’t tell the working girls from, you know, the working girls.
You say that like it’s a bad thing! 
The working girls down on Two Notch Road could learn a thing or two about presentation from their Eastern European sisters! (Former Columbia, SC resident here.)
Hungarian girls can’t help it if they’re hot. And I would say that even if I hadn’t married the hottest one of all.
To the OP: this in no way implies that baring your tummy equates with being a woman of easy virtue. If you like how it looks on you, do it.
To the OP
"What not to wear"had an advice for girls with long torsos. Layer two tops. One long one in a neutral tint, one short one for color. Two tank tops looks especially nice that way, or a long top with a cropped jacket.
There you go OP, if you’re a hot mama MILF you’re fine. If you’re an average human being GET THEE BEHIND ME SATAN.
Ah, yes, Budapest. When I first moved there in '98 (I moved onwards in '03), it was the summer, and I thought Hungary was in the midst of some fabric shortage. I was amazing what was considered appropriate office wear there. As a young 20-something male, it was like being in some kind of adult Disney World, but I wouldn’t exactly look towards Budapest as a barometer of what is tacky or not.
It is expected if you are a plumber.