Aircraft cargo holds are pressurized to the same level as the passenger area, otherwise an enormous strain would be placed on the bulkhead between the passenger area and the cargo hold (ceiling of cargo hold/floor of passenger area). The drop in pressure between sea level and 10,000 feet is just under 5 psi, which doesn't strike me as significant for a bike tire. I suppose if your rims were damaged by sharp objects caught in the brake pads you might run into trouble, but that could happen on the ground, too.

Last year I took my bike on a plane to Florence, Italy and back, and Lufthansa made me deflate the tires both ways. When I got home I e-mailed them saying that I disagreed with their policy. After a phone conversation they agreed to research the matter. A few days later I got a phone call from Lufthansa saying that deflating bicycle tires wasn't necessary, and that Lufthansa would change their policy and allow fully inflated bicycle tires on their planes.