No letter is required or even suggested. (Nor a sign for that matter.) Your motivation on the letter would be service, not compliance:
Effective XX/XX/2018, a new federal regulation requires us to obtain additional information whenever we open an account for a legal entity customer such as a corporation, LLC, or general partnership. In order to avoid inconveniencing your representative, we encourage you to call us in advance when you are planning to open a new account of any kind so we can provide you with a list of necessary information and documentation.
We apologize for the inconvenience to you. This requirement does not reflect a change in our policies. The federal mandate affects all banks opening these types of accounts.
Whether you send a letter or not, every bank should make up a checklist that it can distribute or link on a website for those who arrive without the necessary information or call in advance. The checklist will be driven by the requirements of individual banks, not just what's on the stock Appendix A. For example, some banks will require the individual opening the account to provide his or her own identification. All should require a resolution from the entity's governing body to open the account.
From the input I've had from banks that initiated "early" compliance, there is very little pushback on the disclosures themselves, but people are routinely annoyed that they had no advance notice of the change when they have to make a series of phone calls or a second trip to supply the information or photocopies of identification.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.