All states have disclosure laws that have to be adhered to when selling a home. Full disclosure of any material facts must be disclosed. If any fact is withheld that can later be grounds for legal action later.
You legally cannot conceal any information from your potential buyer that has a bearing on the value or condition of the house. Best to fix all and tell all.
In most cases it is inevitable that you would have to mention a fire if there has been visible damage to the home. If some repairs has been done to restore the home that should be worth mentioning too as it basically removes the effects of the fire on the home.