This is written for my dad, the most ethical person I've ever known...
There is a point in being frugal that crosses the line into the unethical, and it's important for us to stay aware of that line.
I used to have a friend who owned a restaurant, and I was appalled at the things people did. Stealing a dish or silverware in a restaurant is still stealing, however petty, and each time someone does that, a business owner has to include that cost in calculating the costs of services, so we all pay, indirectly, for that theft. The same goes for towels in a hotel.
And smaller things... Yes, when you go for fast food, they expect you to take napkins and packages of condiments. And it's reasonable to take a LITTLE extra---two or three napkins per person in case someone spills a drink (possibly an extra couple beyond that if you have a small child), an extra package or two of salt, pepper, or ketchup in case one turns out to be empty or gets dropped. Most places give you plenty of these when you get takeout (one close to us tosses in 4 or 5 napkins with a single burger), and I put away anything that's leftover for later use. That's just being thrifty.
But I have encountered people who grab a big handful of napkins when buying fast food so they don't have to buy them themselves between visits. The same with condiments. And that's crossing the line into the unethically frugal. Every customer of that business will pay, even if it's only a fraction of a penny, for each time one person does that. And it adds up. My friend with the restaurant said that as much as 5-10% of her overhead was the result of theft. Think about that. For a $10 meal, $1 is paying for theft by fellow customers.
Another example: It's perfectly reasonable to use the restroom at work just before heading home. In my case, although my drive home is usually less than 5 minutes, I always stop because more than once, I've had to wait 10 extra minutes on a train or go the long way due to an accident. I dunno about you, but an extra 10 or 15 minutes can be uncomfortable if I needed to go when I left work. The fact that it probably saves a little on utility use and TP at home is a fringe benefit but not the reason I do it.
But taking a roll of toilet paper or stack of paper towels from work so you don't have to buy them at home is unethical, however you may feel about your boss.
There are also things where the line can be vague and depend entirely on your own comfort level. Very few people will pick up a penny in a parking lot then walk into the store to hand it to the cashier (very few people will bother to pick up a penny at all these days). Many would probably turn in a twenty or a check (though this sort of thing tests people's honesty). Some, but fewer, would turn in a ten. Most would not turn in a one, I think. I think the fuzzy dividing line for most people would be a five.
I probably wouldn't turn in a one, simply because it's very unlikely that someone would come back looking for that amount. I would turn in a five if there was a logical place to do so-say it was in the parking lot of a gas station or small store, but maybe not in the parking lot of a mall. A ten I would definitely turn in, unless there was no logical place (it was blowing along the road, not near any shopping or home). A twenty is an amount I would consider reporting to the police. I would report even a one to the police if I found it about the same time as some incident it might be connected with.
The line is obviously somewhat subjective for any of these, and will depend on your own sense of what is ethical. But ask yourself about that line any time you think about something in the name of frugality that essentially takes money from someone else's pocket.