My rule is, clean grates thoroughly of bits of food matter, and not really worry about simple smoke discoloration. But, as to the interior, if it sticks, leave it. If it flakes, brush it off before it falls on the food. If it causes an air leak or makes a vent stick, clean it off before it makes matters worse.
Doubtless you have read about the "new WSM syndrome", which states that a WSM that has yet to build up a layer of soot on the inside surfaces runs hotter until that heat-conducting soot layer coats the heat-reflecting porcelain enamel-coated interior. I'd say that observation has more to do with a new user's reluctance to really dial those bottom vents down to a state of barely-openness in order to achieve a target cooker temp.
I know I'm sounding sort of self-contradictory here, but I guess each person needs to clean theirs as much as they feel comfortable with or necessary-- just employ good sanitation practices on the grates in any event.