You should use the trademark registration symbol if you have a registered trademark. You are not required to use the registration symbol, but there is a major upside to doing so.
Requirements for use of the registration symbol
You may only use the trademark registration symbol if you have an official trademark registration certificate. You can obtain an official trademark registration certificate from the U.S. Patent and Trademark Office (USPTO). The (R) symbol serves to inform others of your registration. However, you cannot use the (R) symbol if you do not have a registered trademark.
Benefit of using the registration symbol
Under the current state of the law, use of the registration symbol establishes that the defendant in a trademark infringement case had constructive knowledge of the trademark registration. In a trademark infringement case, a trademark owner will forfeit the right to recover lost profits and damages unless the trademark owner can establish that the infringer had knowledge of the trademark registration prior to the infringement. Proving actual knowledge can be extremely difficult. However, the trademark owner can establish constructive knowledge if the registration symbol was displayed in connection with the trademark. Even if the infringer did not have actual knowledge of the registration, it is reasonable to conclude that the infriger had constructive knowledge. In other words, the infrigner cannot feign ignorance to avoid a finding of trademark infringement.
Takeaway
Use the registration symbol as soon as possible! But remember, you cannot use the registration symbol until you secure a trademark registration. Prior to securing a registration, you can use other symbols to notify others that you believe you have rights in your brand. However, those rights are likely far more limited than the rights obtained through a trademark registration. so, register your trademark and use the proper symbols to maximize your rights!