Wheels and tires need to be balanced and optimized as a single unit. No tire is perfectly round. They are all slightly different, just as each wheel can be slightly different. The industry measures these slight variations in 'roundness' as radial run out.
The run out of a wheel is fairly basic. They put a gauge on the wheel and measure its slight differences (or run out) in its circumference. This way they can find the low and high points of the wheel. Today, tires are measured by their radial force variation instead of the traditional weight variances. This means they measure the tire for the force it exerts on the pavement instead of a free spinning wheel's weight. This force variation measures what you feel, and in the end should give you a smoother ride.
Tires come with with paint dots marking that high force variation point. Tire shops mount the high force variation dot to the low point of the wheel optimizing the package and hopefully using as little weight as possible to balance the package.
Now i know what you're saying. "I scrubbed that dot off months ago, how am i going to force variation balance my wheels!?"
Don't worry my fellow detailing nuts there is hope.
Companies like Hunter who make balancing equipment are into this force variation technique and make machines that can find the wheels run out and high force point on their own. Their Road Force Balancers mount the tire to the wheel and push it against an attached drum simulating the pavement and feel for that high force point. This way the technician has all the information they need to give you the best balance possible.
Next time you get your wheels and tires balanced, spring for the Road Force Balance and be a smooth operator.
For more information about match mounting tires check out this tech article that clears up some myths about those paint dots and their locations:
Match Mounting to Enhance Tire and Wheel Uniformity