If you are using NiMH and have a fuse then would have not problems. I have seen NiMH to explode (because the guy forgot he was charging without timer or temperature sensor) and just they liberated a gas... poisonous of course.
I have my NiMh and NiCd batteries inside the tanks, discharged.
I have a lot of respect for Lipos, mostly the big ones, more when you don't have used them for a while..