I was going thru your submission requirements and was amused to see that you recommend never using italics, only underscores. In the 21st century, a manuscript that comes thru production with underscores is more likely to be set with underscores. (I'm a freelancer with 30 years of experience in book production.) Personally, I'll never understand the preference for underscores (blocking descenders); any proofreader or copy editor worth anything knows italic means italic. Furthermore, anyone who can't see that italics are italics is not doing a decent job in the first place. I mean, how can you notice whether the correct letters are being used unless you're reading letters (and I'm referring to individual characters, not correspondence).
Actually, if you wish to give a proofreader or copy editor a more difficult time, Courier or any sans serif would accomplish that (not that I've seen any manuscripts in the past ten years come through in Courier).
Thanks for the comments. I have to say they took me a bit by surprise, since you mention having thirty years in book production. Using an underscore to indicate italics has certainly been around since long before then and the primary reason for it is to ensure that the typesetter knows that the word is to be set in italics, not the copy editor or proofreader.
Just as a pound sign (#) is used to indicate a line space, so that the typesetter knows to actually put in a line space, underlining words you want set into italics helps ensure that the typesetter knows that there was no error. It wasn't an accidental insertion of a line or italics.
Further, if you are familiar with The Chicago Manual of Style, you know that the punctuation following a word set in italics is also italicized. And in some cases, quote marks are also. But it can hard to discern—especially if you are working on paper—whether or not some characters are in italics or not. Using the underscore makes it immediately apparent.
When I go through the electronic version of a client's work and globally search and replace italics with underscore, I'm nearly always surprised by the random spaces or characters that are actually set in italics. Not to mention the sometimes thousands of extra spaces, line spaces, tabs, etc. And in a day when publishers often start with an electronic copy of the manuscript, it's important to send in a clean ms, without all of that extra garbage. If the publisher prefers italics to underscore, it's an easy global search and replace, but when I'm reading things or preparing submissions, I always go with underscore for all of these reasons.