It's stealing. I may be slightly biased here though, as I work in a supermarket. However, I've held this opinion for years, to be fair, while I've only been at the supermarket for about six months. Also, the supermarket I'm at is one where you can sample some things if you ask the staff and it's certain items (for example, we have a Wine Specialist, who you can ask for advice on buying wines with, and they may allow wine sampling for customers - same for our deli counter and cheese counter). I work at Waitrose, by the way. And also note that because as a company we take the quality of our food extremely seriously (we have to, it's literally part of our entire sales pitch), we operate on instructions to remove from sale any item that looks to be in bad quality - something most supermarkets don't do at all.
But yeah, the point still stands. Even though we offer sampling sometimes to customers. If you're taking something before paying for eat, and actually consuming it, then unless that's how things work (like in a restaurant) then it's stealing. Regardless of intent to pay later. If I saw someone at our store eating a grape, or anything for that matter, without buying it first, then I'd be straight onto them calling them out on it, and would get a manager involved too. It's that simple.