Interesting, Rolex, I've never looked at it that way. Side slipping is certaintly not an HO, more of a side-shot, I suppose, as two aircraft slip past each other.
However, I don't see how shooting the underside of an opponents' aircraft could be considered a Head On, unless the opponent "thinks" you are going to HO him/her, and pulls up early...but that is not an HO, in my opinion, it is a judgement error on the part of the opponent.
My basic rule is this: if an opponent points his/her nose at me and I think they have a shot, I will shoot. I don't care what you call it, an HO, deflection shot, side-slip, or whatever, I will shoot. Most of the time when I apply this rule, I die because I've made a mistake in my flying and SA; the other times I get called a HOer. <shrug>