Speaking from experience with AP style, MLA style, and ABA conventions, it's not really that ironclad -- a few official stylebooks have switched over to single-spacing, but in terms of what's likely to actually come off as professional in writing, it varies widely based on the context, industry, region, and country. No such thing as objective language, after all. So rest easy, you're far from the only person schooled primarily in the order of the double-space.