With 200 Bars on the inside of the pipes, the odd half a bar reduction on the outside at altitude doesn't make much difference to the chance of a leak Jack. Its the risk of temperature cycling between very, very hot and very, very cold in the wheel wells buggering up the seals that concerns us. If a seal goes and the lines are pressurised, out goes the fluid. Plenty more hydraulic lines under permanent use are out in the cold, but the temperature variations are not so great and their seals are at less risk. Its always wise to shut down anything thats not being used, just in case.