Where I buy my tires the give me a free rotation and inspection twice a year. My trailer is 9 years old and has worn out 2 sets of tires as I fish close to 100 days per year.
The last time I had the tires rotated the guy said by the December rotation I should plan on replacing the brakes. This is a 20' tandem axle trailer with brakes on the back axle carrying over about 3000 #s of boat, motor, gasoline and gear.