We have a rack that we use when we run out of space inside the car (sometimes we travel with 2 single bikes, 1 tandem, sometimes with 4 single bikes with 4 people).

Until we run out of space inside it's relatively impractical to put anything on the roof. Mileage drops dramatically (5-10 mpg on the current car), you have to think about clearance, if there's anything loose with the rack you can ruin your car, rain/wind/stones affect your bike, etc.