Either by department mandate or by personal preference. Do you wear your hat and more importantly...do you enjoy it?
At my department we have the option to not wear it. I know of only 2 people in my department who wear their hat on routine patrol and on calls. My Sgt. and myself.
That this is freaking pimp. I love my hat. It makes me look taller than I am and just adds to the uniform. Plus, it is custom fitted and the only hat I have ever had and enjoyed wearing. I mean...it actually fits my huge ass head.
Plus, when I go to writing tickets people can't use that as a defense against me :D. I'll throw some pictures up someday with me in my full uni, hat included. Because hell, it isn't a full uniform without the hat!