Using "TM" for trademarked term - every time or just once?

From A Guide to Proper Trademark Use for Media, Internet and Publishing Professionals (pdf), a publication of the International Trademark Association:

Generally, demarcation is not necessary for every occurrence of a trademark or service mark in an article, press release, advertisement or on a website, etc; however, at a minimum, this identification should occur at least once in each piece, either the first time the mark is used or with the most prominent use of the mark. When in doubt, err on the side of “over-marking.”

"TM" should be used once, with marks for which trademarks have been applied for and not approved.

"®" should be used (once) for registered trademarks.