I'm about level 17 or so on Pocket Planes, and I have been mostly opening class 2 and 3 airports now that I can afford to. I'm noticing that I have a ton of jobs now that I can't fulfill; I only have 11 airplanes. Does it make sense to keep class 1 airports open, if they're just going to generate jobs I can't fulfill? I can make more money with larger airplanes going to/from the larger airports anyway.
Of course, some of them would remain open as waypoints between cities so that the smaller planes can still fly from city to city.