Even though stump grinding is fairly straight forward, I wouldn't do si without. A contract protects both the contractor and client. A couple of examples:
For you:
*Helps ensure payment (or at least puts the law clearly on your side if you need to go after it)
*Places responsibly for underground utilities location on client
*Limits -in writing- a definition of what you agreed to do, and what you didn't agree to---so they can't say "you were supposed to clean the chips, replace it with topsoil, and bring in sod" when all you agreed to was a simple grinding.
For the client:
*Shows them that you are responsible (insurance) for unexpected damages.
*Ensures they know what they are getting -- so they know if they want the full service, they can show you in writing what you agreed to.