Nearly everyone does use the "Ask HN" prefix. Based on http://news.ycombinator.com/ask, 87% of all self-posts start with "Ask HN" or something similar. And the prefix's use is trending upward over time.
New users really have no idea that is the typical protocol on HN; I know it took me a week or two to notice things like that and then dig up the posting guidelines.
Might be worth having a quick FAQ show up when a user creates a new account to cover the basics like this.
More to the point, why does it matter?