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
When your ROM is booted up for the first time, Android's Dalvik goes through every apk and extracts the dex file from it.  It places the dex files in the cache folder - located at /data/dalvik-cache.  This is the directory that gets cleared when you clear the dalvik cache in your recovery tool