Do I need to let air out of any or all of them before I fly?

pump them up really hard

No last time I pumped the tyres up more so I could get to the right pressure by deflation when we arrived as we weren’t bringing a track pump. Airline didn’t care.

I’ve usually let my tyres down to fit the wheels better in the bike bag. Left everything else as is although I’ve mainly flown with coil shocks and forks.

The hold is pressurized on all modern aircraft. If it depressurizes, your tyres swelling up a bit is the least of your concerns.

Release the same amount of pressure as the captain releases from the planes tyres.