I make potatoe bags and yes they are used to make baked potatoes in the microwave... I personally havent had any problems of them catching on fire, I beleave washing the fabric is a very good idea, I always wash my fabric before I make a bag. I beleave allso that if a bag catches on fire it may be due to the microwave voltage, some cooks at a higher heat then others.
Just my personal opionion.
Janet