Map Completion - Should I do it?
Well they used map completion as an example of retroactive mastery points so if you want the points go for it. It’s basically guaranteed. Even if they hadn’t said anything, its a safe bet that it would be the kind of thing to award mastery points. There is no reason to wait
Guardian / WvW Enthusiast
I say there’s no risk at all. I cannot imagine they would deny those one-time mastery points to accounts that have already done world completion. Remember that every two legendaries require a map completion for the Gift, look at how many legendaries you see in game, and then consider blocking a mastery line for everyone that’s made them … nah.
Most definitely retroactive! And besides, they know how to do it. When they introduced achievement chests, people got a chest a day until they caught up to their number of points previously gained.