It isn't a matter of appearing amateurish, it's a matter of what agents, editors, typesetters, etc., are trained to look for. As an editor, underscores would piss me off, though I'd never mention it to the writer. (Editors seldom mention anything to writers, unless teh format is completely unusable.) If the writer underscores, I have to go through a print manuscript with a pen and underline everything to make sure it's spotted, and if it's an electronic file, I have to do the same thing electronically.
Seriously, if you're losing HTML format because of underlining, or working harder because of underlining, you need newer technology.