If, after 30 days, you haven't chosen and selected a Most Helpful opinion, one will automatically be chosen for you. Neither the Question Asker nor the user awarded Most Helpful will get Xper points for these automatic selections. It's chosen based on engagement: votes, comments, length, time, xper level, gender, among other criteria.
After around a Month or so the "Most Helpful" answer gets chosen automatically by the system. You do not get xper points for it and the answerer doesn't get xper points for it either. It is selected based on upvotes, length, xper level and other crap like that. So I'd suggest you to award a M/H before the systems randomly chooses it.