I did a Google search of stories going back to 1995, one year at a time, with the filters +police +dog +shot +fear for their +safety. I wanted to see stories that used the phrase "feared for their safety" as the excuse used by police who shot dogs.
What I found was that the first police-shoots-pet story was in 2001. this phrase began to be used in 2003, and increased in number as the years progressed, especially in 2007, 2010, and 2011.
Here is a link already encoded with the search terms to see when the excuse of fearing for the life of the officer became more prevalent.
-PJ
Since this is such a critical subject perhaps you could establish a national database of all known dogs that have been shot. That way those that are interested could get accurate stats.