[Al Skierkiewicz]

cheese,
The batteries are specified in Amp Hours. To discharge 100 watt hours would be to drain at 8.3 amps for an hour or about 0.5 C. While this is not a harmful or beneficial discharge, it alone is not going to extend life.
The 10.2 volts represents the manufacturer's test cell voltage of 1.7 volts/cell. This is the point at which the manufacturer tests the AH ratings at various discharge rates. It will not be damaged at this voltage loaded. It is at this point that the cell voltage rapidly starts to nose dive towards zero volts. (The corner on each of the discharge curves.) For the NP18-12, self discharge to 50% rated capacity is greater than 12 months at 20 degrees C, longer if it is colder (within reason).
I am not one to recommend open circuit voltage testing to determine battery condition. Open circuit does not measure increased battery impedance due to other factors. The Battery Beak or the CBA III both test at some load to determine battery internal impedance and therefore actual health. Our batteries should test at about 0.011 ohms. I have seen batteries that test fine open circuit only to later find reduced cell capacity due to broken plates internally. A fully discharged battery will after several minutes, actually measure 12 volts. These will even under load show fine for a while and then suddenly drop 2 volts when the defective cell cannot supply any current. Search for one of my posts from a few years back on this subject. I posted comparative curves for good batteries and defective ones.