Should I keep my MacBook Air plugged in when not in use?

The charger in your MacBook is a smart device so it is unlikely to cause any significant problems. Keeping the battery at 100% for extended periods is not ideal and might have a very small impact on long term life, but you have to balance that against using and recharging from a reasonably finite number of charge cycles each of which makes a minor reduction in battery life. There is no evidence to support either scheme, but I have never had a battery fail from being kept well charged. I have had batteries fail because they were allowed to discharge fully and not recharged promptly.