John lets himself pause and consider the question, though the answer feels obvious enough to him: it's there in the immediate pang of disappointment he feels when Martin starts to lift his hands away, and the almost petulant conviction that he's hardly had the chance to enjoy it, yet. But Martin is right to ask, and John doesn't want to respond entirely off the cuff. So he gives himself the opportunity to imagine the potential repercussions, both good and bad.

The good seem fairly self-explanatory: he loves it when Martin plays with his hair, and he thinks Martin might have a more pleasant time of it if he has something to do besides 'try to hold still.' The bad would likely be no worse than Martin accidentally pinching him during a moment of heightened distraction, which wouldn't be the end of the world — and even that risk seems negligible. Odds are Martin will take extra care with him, given how new this is. Hell, if he manages to make Martin forget himself so thoroughly that he causes any discomfort by mistake, he could probably take that as a sign of a job very well done.

So John smiles faintly, and says, "No, thank you," before leaning in to give Martin a brief peck. "I trust you'll behave yourself." One of them ought to, after all, and it certainly won't be him. He presses a parting kiss to Martin's chin, then moves to reposition himself. This time, in the interest of equity, he intends to give Martin's left side more of his attention, so he lets his right hand take up the mantle of holding Martin's undershirt up out of the way, and his left hand shifts to cup Martin's right side.

Best to start slow, he thinks, while Martin adjusts to his new privileges. So he doesn't quite make contact, his left hand giving his chest a gentle squeeze as he sucks a faint mark over Martin's heart.