I answered
I have drank/ate something before purchasing AND I don't think it is ethical to do

It has happened that while at the grocery store, I was feeling hypoglycemic, so I've eaten a few grapes from a bag of grapes I had in my trolley. I usually tell the cashier that I've eaten a few grapes when that happens, not that it bothers them. The overall weight and price is usually the same. It's usually best to wait to pay, before nibbling on what you've just purchased. I do also believe however, in the case of an emergency, like hypoglycemia, which can cause nausea, dizziness or fainting, it's best to grab a quick something before the worst happens, so that you can be conscious and well to pay for it ;)