Batteries are chemical products which internally undergo certain chemical processes over time. Due to this process some batteries slightly discharge during storage.
In appliances without a charging function, yes. In appliances that do have a charging function, be sure to check whether the function was specifically designed for Ni-MH. If not, it’s best not to use Ni-MH batteries.
Never apply heat to batteries. Keep them away from any heat sources like heaters, candles, stoves and fires. Exposing them to too much heat could cause them to tear, leak or even explode.
While leakage is rare, it may still occur if the battery is used incorrectly. Leakage is usually caused by leaving the battery in an unused device or short-circuiting.