Underlining is another holdover from the typewriter age. Typewriters had no bold or italic styling. So the only way to emphasize text was to back up the carriage and type underscores beneath the text. It was a workaround for shortcomings in typewriter technology.The Maroonbook agrees, but I'm not convinced. I still like using underlining for case names. There's something elegant to me about Marbury v Madison, 5 U.S. (1 Cranch) 137 (1803), which isn't as apparent when formatting it as Marbury. Moreover, I find many more problems with italics in terms of accidentally italicizing spaces and punctuation marks which are more easily thwarted with underlining. When you underline, you know exactly where the formatting ends.
Your word processor does not suffer from these shortcomings. If you feel the urge to underline, use bold or italic instead.
Not convinced? I invite you to find a book, newspaper, or magazine that underlines text. I notice it only in the tabloids. Is that the look you’re going for? No, I didn’t think so.
Related question: if you underline case names, can you still italicize other things in the same document like See or de minimis?