Re: To Slap the Ground or Not
In general practice, I don't think it's dangerous at all. It does seem to help ease the fall some.
One danger does come when uke reaches for the mat and lands hand/wrist first.
As for in a self defense situtaion, then yes, it can cause you a problem. If you are going down, you need to be using hands to continue to defend yourself. For an example, even though it's not a "self defense situation" watch high level Judo competition. What happened to all the mat slapping they learned in the dojo? It gets thrown away when they are focused on winning. They are using thier hands to try to regain control over thier opponent.
In a Judo competition, slapping will cost you the match. In the street, slapping out can get you killed.