burnable cd's and dvd's degrade over time. Sunlight will degrade them much quicker otherwise you need to reburn them every few years unless you are using the more expensive archival quality disks and then its 50 to 100 years depending on the cd's/dvd's.
on stamped disks, like music that you bought from the store, keeping the backside scratch free is much more important than the clear side. The clear side actually focuses the laser onto the reflective side so light scratches there don't have a huge impact. But a scratch on the reflective (label) side of the disk is like the grand canyon to the focused laser beam. I once had a customer engrave his name onto all his disc's on the label side and wonder why they no longer worked.