I think it's best practice, even if not written. As far back as I
can remember, you couldn't access ftp.uu.net without a proper reverse
and that's probably going on 14 years or so. The relevance there is that
it was considered proper that far back all the way to this day. Many
services won't respond or allow without reverses either.
As far as charging, I've seen ISP's set limits on how many changes
a month. Not on establishing the records. They're a required part
of the service - in my mind.