I am having a debate with a vending company of ours on ACH transfers. Is a financial institution required to send out pre-notification for recurring debits or credits (once every 30 days)?

No pre-note is required. It is an optional item per NACHA rules, under the Operating Guidelines, Section III, Chapter I, Part B #1 "Use of the prenotification process by an Originator is optional for all Standard Entry Class Codes."

Every seminar that I have ever been to says we have to send pre-not. Maybe that is just the opinion of those teaching the seminars.

It could save on non-posts or fixing on the receiving end before a live transaction would get sent out, but there is no official rule saying you have to.

My bank errors on the side of caution and sends out cards requesting the customer's signature as authorization. Sometimes we actually catch unauthorized ACH transfers before they hit the customer's account. It also helps us prove to some customers that it wasn't an unauthorized transaction if they complain to us later on.

It's my understanding, as well, that sending an initial pre-note - while a good idea - is not required.

Interestingly, however, the bank that receives a pre-note must verify it per ACH Rules.