Never let air out of my forks when travelling, never had a problem, been asked to let the air out of my tyres before now which is pointless too IMO.
Having said that… its all very well to have a bit of paper to wave about, but its the staff in the airport who will decide regardless of what your bit of paper says.
We had a punch up recently with Easyjet when they briefly changed their bike carriage from 32kgs down to 23kg we booked and had documents to show that we had 32kg allowance – they changed it to 23 a week before we flew.
They completely ignored our paperwork and protestations and wanted to charge us a fuckton of money for the extra, I insisted they checked the notes on our booking – luckily someone had specified that we were indeed allocated 32kg so everything calmed down and we went on our way.