Referring to writing novels (and other fiction) only (i.e. professional writing), italics can be used to indicate a foreign word or phrase such as
déjà vu, as a signal to the reader that the phrase is not English. Italics can also indicate a "made up" word. Italics are also sometimes used to indicate thoughts (of a character). In fantasy it is not unusual to see telepathic communications set in italics.
Bold should be used only for chapter or section headings.
I cannot think of any instance where underlining should be used.
When writing for novels no other formatting should be used, and word processor should be set to defaults including default style. The publisher will select font and point size during preparation for printing.
In writing my first short story I kept feeling the urge to insert smileys!
As far as forums go, everything is the wild, wild west! "We don' need no
grammer [sic] here!"