My machine shop told me that the valve guides on my head (d16z6) are worn and need replacing. They also said it was MANDATORY that a valve grind be done. They want to charge an extra 300 for the whole process. My question is, if I pick up a new set of guides, and have my friend mechanic install them, is it absolutely necessary to do a grind? Sorry for the noob question, but for some reason I don't completely trust the machine shop, because they want to make a sale..