I've lived in King of Prussia for over 20 years and have only had one experience with "crime" that was of significance and that was some kids (12 year old girls) spraying graffiti on my house while I was out of town. All the neighbors who were home came over with pressure washers, brushes, paint remover and elbow grease to help me clean it off.
As for your question about safe areas, Fair Housing states (they consider it "steering" if I answer your question) that I can only recommend you contact the Upper Merion Police Department at 610-265-3232.
When I moved here 26 years ago I found the area fascinating with so much history, places to go and things to do. Feel free to contact me anytime if you have any questions.