This tip is quite common sensical.  If there is any ember left in the ash that is being removed from the heater it is still a fire danger.  It is always a good practice to wait until the ash is cold before doing anything with it.  Regardless, it is a good idea to use a metal container when removing ash from your heater or cookstove.

Clean ash from a heater fire can be a good soil additive, so go out and spread it under your fruit trees (or on top of the snow under them!).