Just because Gary Halbert buys his first PC in 1995 and suddenly changes his font from Courier to Times he "thinks" (believe me, there was NO "thinking" there) it's OK to underline Times because Microsoft Word allowed him to do it.
There has been NO UNDERLINING of typographic fonts (all fonts other than "typewriter" fonts) by those who understand the rules of typography which have been in place for 500 years.
If you have WOOD to underline, then use Courier in your sales letters.
First of all, understand that there was never any underlining in typography. NEVER. (Underscoring, but not underlining. . .there's a HUGE difference.)
For a huge chunk of the 20th century, typewriters were used for business and personal communications. There was no way to italicize using a typewriter so UNDERLINING took its place.
Computers allowed anyone to use any font and they were now giving the ability to BOLD and ITALICIZE fonts just like typographers whenever it was appropriate.
Underlining was only done on a typewriter. Some offline gurus who moved over from typewriter to computer didn't understand many. . if any of the simple rules of typography. (Like putting one space after a period.)
DO NOT underline any font other than a "typewriter" font. Doing so makes you look like a STOOPID NOOB.
The only style manual I've seen where underlining, a font like Times, is required are for LEGAL court documents. Somehow they've moved over from Courier to Times and kept the underlining.
Show me any book, magazine or newspaper that has underlining in the body copy of a story or ad. You can't. (Unless it's some internet guru or NOOB dipping his toe in the offline world.)