Thanks for input so far!
I'm not afraid of fires, only about losing valuable belongings that can not be replaced - such as old photos or paintings. Because the fireplace often is positioned on a central spot in the house, like an alter in a way, it also works well as a natural spot to expose photos and what not.
I take it that it would be better removing those family photos from the fireplace, keeping the urn there but rather add some flowers or general decor. Hm, I would never place a vase with flowers on top of a fireplace due to the temperature would kill the poor flowers, but then again we don't need to be toooo realistic in the sims' world :)
In my game I focus very little on skills, especially the mechanic skill. In a house I just played I even discovered that a large floor carpet was placed *under* the fireplace, obviously a mistake due to bb.moveobjects being on. Still, nothing of this caused fires. I might have seen 1 or 2 fireplace fires since game release in 2014, not all my houses have a fire place, though.