After I posted above, I searched some facts of LiPo on the net;
- 100% charge is danger because rising temperature causes battery "over 100%" charge state,
- natural discharge is faster in 40oC than 20oC, 20oC than 0oC (but monthly scale),
- degrading (yearly scale) is worse in 100% charge than 50%, 50% than 0%,
- the irreversible damage is shown under -20oC due to some chemical material freeze (but other paper says "space LiPo" could be kept under -70oC...?)
So the best way to keep battery is, 0% charge at 0oC degree; but very slow overdischarging occurs and battery may be damaged, so need to charge slightly to 0% once in few month. Many R/C shops cannot maintain like that, so 50% charge and keep under 20oC is the 2nd best way. In hotter region, keeping in vegetable partition in fridge (5-10 oC) is good way. And be careful avoiding dew when take out.
Back to the topic, battery damage will be occur when over 40oC AND 100%. Fully charged battery is not damaged when just after starting motors on 25oC, nor med in flight 50oC but 50%, you know. Just after landing and remaining battery 0% or 25%, rather safe even if you do "force charge" (power on then charge)... but should stop under 75%, then wait till battery becomes cooler. Instantly, obeying charger warning is safer.
I sometimes fly on morning, connect to charger (warning and doesn't start charging), leave and go to job, then charge automatically starts when cooled and fully charged when back home. And if, you're in hurry, force start charging is only the way - travel to seaside and shoot sunset scene is more precious than battery life.