Its an interesting question, and in some cases hits environmental concerns. Some airlines wrap their blankets in plastic so you know they are clean. But of course thats not ideal environmentally. I sometimes wonder about outstations and cleaning, not suggesting Air NZ specifically. I guess one can always ask the crew if items have been cleaned? Though of course its not like the seats are cleaned between flights (or ever!) so I guess the difference between a pillow and seat back/ arm rests etc might be marginal.