I have read and learned so much lately that I am now thoroughly confused. So, someone please answer this very basic question about your basic 4.8v Nicad/Nimah batteries. Regarless of charger, regardless of battery - When it reaches full voltage (i.e., 4cells x 1.4v = 5.6v more or less) is the battery now at full charge now matter how long it took (i.e., 5 minutes or 15 hours); and leaving it on the charger beyond this point serves no purpose and may actually damage the battery.

You are correct, once fully charged leaving it on charge longer serves no purpose and can be detrimental to the battery.

I'm afraid it's not as simple as all that. In the first place, the voltage of the pack during the charge is dependent on the charge rate. The higher the rate the higher the voltage. It also depends on the cell construction and the manufacturer. I recently graphed the charge voltage on two 700 mAH NiMH packs at a C/2 charge rate (350 mA) starting with fully discharged packs (.9 Vdc per cell with C/5 load).
The Sanyo pack was reading 5.6 Vdc after 10 minutes and reached a peak of 6.19 volts at 120 minutes indicating a full charge.
The Panasonic pack started at 5.7 Vdc and reached a peak of 6.59 Vdc at 110 minutes indicating a full charge.
If I had been using a higher charge rate, say C/1 (700 mA) these voltages would have been even higher. So forget the rule of thumb stuff, every pack is going to be different. This is why the peak chargers do not terminate charge at a specific voltage but at a specific rate of change or a dip in voltage.