Should I wait until my carpet looks obviously dirty before I get it cleaned?

No, outdoor air contains pollen’s fungus, bacteria, air pollution, cigarette smoke, car fumes, and hundreds of chemicals.
When you or someone else enters your home, all those pollens, bacteria, chemicals and pollutants come in with you on your skin, clothiers, hair and shoes.
All those chemicals, pollens and other pollutants and bacteria end up in your carpets.
If you or someone in your home has allergies, asthma or other breathing problems – a major contributor could be these pollens, bacteria, chemicals and pollutants inside your carpets.