Social Dilemma - Is It Okay To "Taste Test" Grapes Before You Buy Them?

Had this conversation over the weekend with some friends and I thought it would make an interesting social dilemma.  We were talking about grocery shopping (Exciting, I know) and the topic came up about buying fruit.  Long story short, we started talking about "taste testing" some of the fruit.  Grapes for instance.  Do you try the grapes before you buy them.  I admit it, I've done it.  I don't know if it's right or wrong.  The grapes are open but I guess that doesn't mean I'm supposed to try one, but I have, mainly because I don't want to buy sour grapes.  Some of my friends said they've gone even farther, like trying a plum, or some blueberries.  That doesn't feel right to me, but I thought I would bring it to you.  Is it okay to "taste test" the grapes at the grocery store?  What about other fruit?  Let me know what you think.