I’ve heard that traditionally any italics in a manuscript should be indicated, instead, with an underline. In this modern day and age with computer programs and all, is that still expected. Or, at the very least, does it indicate unprofessionalism on my part if I persist in using italics since it’s the way I hope to read it one day in published form.
A very small point, I know, and, honestly, I’m not obsessing about it. But, in terms of putting your best foot forward . . .
When you query, use italics.
When your book sells, and you are preparing page proofs, the publisher has a style book and they will tell you what form they use. You'll follow that form, at that time.
My sense is that underlining for italic is formatting from the days of manual typewriters, but like double spaces after periods, that has changed with the arrival of new technology.
The bottom line though is that it won't hurt you even if you get it "wrong". If you write well enough you can send it handwritten on the back of a Sardis napkin and I'll read it. Trouble is, most people who think they write that well, don't and most people who do write that well have invested in paper and a printer.