Most batteries benefit from charging for a long period at the "C" rate or 10% capacity ie 1000ma pack charge at 100ma for 15 to 20 hours. I have been told you can plug in a "C" rate charger and leave it basically indefinitely. I still do "C" rate charging in the shop. I have several spare (un needed at this time) packs in 4.8 and 6v and about every six weeks or so I cycle them down and put em on "C" rate for a day.
I doubt if you hurt your batteries with the "wall warts" they're pretty safe as far as leaving them on for extended periods.
Doc