You should use premium if it starts to ping- like high altitudes, towing, etc
the engine has ping sensors so you will not hurt the engine w/ low grade petrol.
I ran over 3 years on the 1GR-FE. 90% of the time on regular octane. I used premium occaisionally (crappy gas stations, was at costco, etc)
High alt, city gridlock, high heat, etc. never any problems or noticed any real difference between premium & regular.
also,my understanding is that regular gas has more "power" per gallon than premium. Premium has more octane which takes more energy to combust, thus it won't pre-detonate. (non-scientific description here, some fuel expert can clarify this)