I'd recommend going against the grain on the back of your head, otherwise you'll be doing multiple passes with the grain, not getting that smooth feeling, and irritating your scalp.
However, I'm not sure how effective the Headblade is against the grain (I use a traditional razor) and it took me months before I was comfortable with shaving against the grain on the back of my head.
And because your Headblade is a Sport model, that means its a pushing, not pulling motion? For that reason it might be easier to go against the grain on the back of your head, I'm not sure. Would love to hear some feedback from those who use the Sport against the grain.