You should consider rotating your tyres so that you get consistent wear from all of the tyres.
Your manual will tell you how and how often to rotate the tyres (which means swapping them between the different wheel positions on your car) but a general rule of thumb should be that tyres are rotated every six thousand miles. Please note, you should not rotate tyres if a tyre pressure monitoring system is fitted to your vehicle.
Remember also, you should not automatically include the spare tyre in the rotation sequence. Some spare tyres are specifically manufactured for emergency and temporary use only; check your vehicle handbook for details.