Not essential but skimming would give a finish consistent with the rest of the room, and cover the drywall screws and any gaps between boards, maybe not strictly necessary for a small section of board.
If you're only talking about a gap between the cistern and the wall, it might be a hundred times less work to just make wooden battens to fill the gap (maybe recessed in from the top and sides of the cistern for elegance) and painted to match the wall or the cistern.
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