a lipo packs nominal voltage is 3.7 volts.. they are fully charged at 4.2 volts per cell... your balance charger will stop charging each cell at that number and give a total of 8.4 volts... You are not overcharging them at this point... more than that will be an issue...

I occasionally let my batteries sit for a couple of weeks between full charging and running... I have alot of batteries and don't always run them all.. they at the worst have gone down to 4.15-4.17 volts.. they don't drop like NiMh or Nicd packs... If storing for a long period.. all winter then as Pinolelst said, drop them to 3.7 volts... or 80 %... One or two weeks is not a long time between runs and it won't hurt anything if you do other than add extra cycles to the batteries...