follow manufacturer suggestions. I drive/have driven several German models and they all require mid or premium. Many other high-performance cars are tuned for higher octane. Most US brands only use regular but my Corvette (older) used premium.
Just follow the directions from the manufacturer for the most part. Using higher octane for cars that don't need it is a big $$$ waste.
-I've found it is more important do an engine flush and to use gas treatments regularly. I have also noted a bump of about 2 mpg when using them.