Proof isn't a condition, but a manufacturing (minting) process. The dies and planchets are highly polished and the coins are struck at least twice. The mirror finish can't just wear away without destroying most of the coin and by that point it isn't worth anything, but maybe face value at the most.
http://www.usmint.gov/about_the_min...rading#ProofThe P mintmark wasn't added until 1980, so dimes before then, from Philadelphia, won't have one.
Edited by wquinn
01/11/2012 10:56 am