On my F flight LHR-LAX I was actively encouraged to go up the stairs and "have a wander round" (although I think I told the cabin crew I had friends seated upstairs, which I did). In fact I went the whole length of the plane on the upper deck and came back to the front on the lower deck. I really wanted to see what the Club World cabins were like too so it was an opportunity to see those cabins on each deck (and confirm to myself I needn't get excited about booking that anytime soon on the superjumbo).