The thing with most NiCD and NiMH batteries is that they can take a fair amount of overcharging at low currents, say C/10 or below (C being the battery capacity in mAh). This means that if you battery is say 1300mAh charging at 130mA or below doesn't hide a risk of dangerous overrcharging. In fact some manufacturers, like GP for instance state that their NiMH cells can be charged indefinately at C/10 or below. I don't really believe this is the best thing one can do, but leaving it in the charger for several hours longer won't hurt them badly.
The things change when you apply higher currents. The temperature and pressure in the cell rise significantly at overcharging (try to touch your walkman at the end of the charging). If the current is high enough, the H2 and O2 formed at the electrodes cannot be recombined, the pressure rises and finally the cell vents (explodes).
Don't worry, this will not happen to your MD. Sony has limited the amount of overcharge. However, charging at this rate still heats the battery and the pressure rises every time you do it. This shortens the live of your battery.
Typically, the live expectance of the modern NiMH batteries providing proper charging and care is applied is anywhere between 500-800 cycles before the capacity drops significantly. This means that with proper charging they most probably will outlive the portable. With the abuse of the fast in-unit charging these batteries last for for no more than 100-150 cycles, go figure. You can fast charge NiMH batteries without a risk, but you'll need a more sophisticated charger that can terminate the charging at the exact point. There are several methods to do this, we can perhaps discuss this in a separate post.
About the full discharge, NiMH have much less memory effect than NiCd, which means that it is not necessary to discharge a NiMH cell every time before charging it. Once every 5 cycles is usually enough.
However, be careful there too. I don't know how Sony designed their charger circuit. If it has only time cut-off (timer), charging not fully discharged battery will mean even more overcharging.
Hope this helps.