As stated above, “never” is the correct answer, unless you are the company itself trying to protect it’s own trademark.
That’s why I commented on **Tomndebb ** in that other thread. Why should a Straight Dope mod use a trademark after Google? Is that an official requirement now of board “employees” or just a stylistic quirk? It would be interesting to know.
But as a private individual you never have to use a symbol when you merely refer to a trademarked product. Nor do you have to use the trademark properly in a sentence, as in Xerox-brand paper copy or Kleenex-brand paper tissue. You can say made a Xerox copy or used a Kleenex or for that matter Googled someone.
Note that it is not the public use of such terms that can lose the company its trademark, but the company’s lack of defense against those uses, as **Chuck ** wrote. That’s why the legal notices go out and why companies put ads into magazines like Writers Digest and the Columbia Journalism Review. They don’t expect anyone to change their practices, but it gives them legal protection. Because of these efforts it’s been a very long time since I’ve heard of a trademark being lost because of public use.
The use of trademarked images is a different matter in many ways. You would not need to identify a trademark merely because you put its image onto your website, for example, but its use in certain commercial contexts may require a trademark notice. Commercial considerations do not carry the same first amendment rights to free speech.
The notion that because the SDMB uses Google Ad Sense and therefore has to put a trademark after the use of Google does not fall into this category, however. That’s simply wrong.