Many dogs occasionally eat grass and remain completely well. Context matters: frequency, vomiting, appetite, stool, energy and access to chemicals or toxic plants. Repeated vomiting, pain, lethargy or inability to keep water down deserves veterinary attention. Keep lawns free of unsafe chemicals and do not assume grass eating is a reliable way to diagnose an upset stomach.
Use this as a starting point
Every dog is an individual. Age, medical history, breed, environment and previous learning can change what is appropriate. When in doubt about health or safety, consult your veterinarian or a qualified professional.