I don't think the original poster was talking about 121.647 fuel. Yes if you know that MIA is going to be blown up by TS then by all means put some extra on. However that extra that you put on for the potential TS is considered 121.647 fuel. Now that is my opinion and my interpretation of that particular regulation. I think what the intent of the original poster was do you put the extra fuel on for the flight if there really isn't a need for it and do you do it because a specific flight crew won't go without because of an act of God in the previous past.
As far as extra fuel into specific cities I agree luigi that we need to take that into account. If you know that can hold it's traffic for a significantly large amount of time for precip, TS, etc then it should be done.