Internal resistance is the factor,
that will determine the overall health of a lithium ion cell.
I typically see the IR values increase substantially after 5 years,
whether the cells are used or not.
IR increase will increase heating of the cell while charging and discharging.
Panasonic & Sanyo cells seem to hold up the best in my testing.
LarryDFW
P.S. Mitro noticed a decline here with 1 year old cells vs. new ones:
Graph for Old vs New