Just beware of marketing cost going exponentially high if you serve as many airports as you can as fast as you can. Once I get big, I can freely add flights to as many airports as I wish but I think early on you must be careful. For example, under statistics for company brand & image, we have people closing in on 80company image while we have people who serve 60+ airports already only at a 30 company image.
If you take a look at me at DFW, we can compare my airline that only serves 20 airports to the other Airline that serves 70+ airports. I have over 35 new aircraft coming in while he only has enough money for 4 new aircraft coming in. While Technically right now, he serves over 3x more place than I do, flies more passengers, has more planes, if you take a look at the actually stat numbers and finances and aircraft on order, it is a matter of time until my exponential growth surpasses. I know a lot of people try to claim their stake in their route by serving the airport. Consider this, my company image is closing in on 80 while my competitor is closing in on 30. Now I can start my massive airport growth. I have plenty of new aircraft coming in(about 3.5 per month), I can add every airport he serves. Now consider this: imagine who is going to get the majority of the passengers, the person with CI of 80 or CI of 30 once I start serving all the airport he serves.
So in the end, i personally believe it is better to Serve the same airport as many times as possible until it is maxed out before I move on to another airport(at least during the first game year)