I get frequent complaints from libraries about patrons getting emails saying their items can't auto renew. The typical error message is "too_many", and it's sent every single day. Most of the time, we probably only want to send email notices out when the item has successfully renewed. Then notices like advanced item due, item due, and overdues take over from there.
(On a side note, the precedence of errors in automatic renewals makes no sense. In the most common scenarios, you'll probably never see an "overdue" because you'll get "too_many" first.)
Since people will disagree on this one, we would wrap it in a system preference anyway.