If you are traveling overseas and carrying US cash, they will have to be fresh, crisp, new bills. Money changers and vendors who accept US cash will often refuse bills that are soft or not the durrent designs. Lst summer I tried to pass a $100 three different places, it was refused because it was the old printing, with Franklin’s portrait in a large oval frame. It was refused by two hotela and by a ticketing agent at Emirates Airlines.
So before you travel, go to a bank and make sure you have crisp fresh US bills. In the past year, I’ve been in two countries that have no national currency of theri own, prices are quoted in US dollars, and they are very fussy about accepting old or worn or soft bills.
Companies that do a lot of mailing tend to use franking machines, which (at least in the UK) give you a substantial discount off the standard postage rate anyway, so it wouldn’t be economical to buy up cheap stamps.
For small-scale private individuals, though, it is worthwhile. I used to sell a few items a month on Amazon, and I bought discount postage stamps in bulk on eBay to save on postage costs.
I keep a stash of beautiful old stamps for freelance-related (Graphic Design) mailings.
A half dozen engravings depicting Westward Expansion in a variety of sizes and muted colors does get attention.
Come to think of it, I got a great Ad Agency job with a handful of antique stamps and a hand-written-and-doodled resume.