If everything checks out good - tire tread, tire pressure, etc - what less than mpg would trigger you to replace your 12v batt?
nothing with an 08, i would test the battery and run through the low mileage checklist to see what else it might be.
I would not change the 12 volt unless it showed less than 12 volt before starting in the morning. http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html