It is okay to feed chickens grapes, though you should wash them thoroughly when you get them home from the store. By doing this you will be able to ensure that all of the pesticides and other chemicals have been thoroughly removed. You will also want to be sure to moderate the amount of grapes you give to your chickens, as they are not a natural part of their diet.
Grapes have a lot of vitamins, minerals, and nutrients that are good for both people and animals. There is nothing toxic in grapes that you have to worry about when feeding them to your chickens, so you can feel free to do so once in a while.