how often for a tire rotation
You'll find tire-company recommendations for rotation that range from every 3,000 miles to 15,000 miles. The best interval is probably somewhere in between, at about 7,500 miles, unless the tread on one or more of your tires is wearing differently from the rest. Michelin, for instance, suggests rotating tires every 6,000 to 8,000 miles. I won't re-balance a tire unless I notice odd wear patterns or, as you mention, feel a vibration.