ODEX file: Does every app has one and if yes where are they stored?

I know that .odex files are optimized .dex files. I have seen a lot of .odex files below the path /system on my phone. I am just curious if there are also .odex files for my installed apps. I searched the file system but I wasn't able to find .odex files for my installed apps. But I assume that they exist - the question is just where they are stored. BTW: My phone is a rooted Samsung Galaxy Nexus with Android 4.1.2