@Fenceman I believe that the 51 PSI is indeed just that a absolute maximum air pressure. Not likely ever used certainly not on our cars. The recommended tire pressure is on a sticker inside the drivers side door jam, that will give the recommended operating air pressure for your C5. I run mine at 30 to 32 on my 03 C5Z.
Some people will bring up the pressure for winter storage, the thought is to stop any "flat spotting" of the tires if your car is stored with the tires on the ground. Tires will lose some pressure over time but unless you have a rim leak, usually caused by rust or dirt on the bead of the tire or rim. Some tires, Michelin for example do not like to be store in very cold areas, I put mine in the basement and used the original tires and rims for storage for fear of cracking the tires themselves. Rims occasionally just need to be cleaned off but that requires dismounting the tires, expensive to do. Bring them up to recommended specs and just keep an eye on the pressure of each tire.
Good luck with yours.