They very well may need replacement but, because they are worn...NOT because of the grinding noise.
It's pretty easy to tell when your pads need to be replaced because the sensors will trigger the light in your instrument cluster to alert you. The disks are a little tougher. Usually they are replaced because they have worn and are too thin. The only way to tell is to measure the thickness with a micrometer.
Any good shop worth their salt will save your old parts and gladly show them to you. Ask to see your old disks and have them show you that they are out of spec. If they cannot do that I would never return there for service.