Leaving them on the lawn becomes a problem only if they are too thick. If you mow the lawn before it gets overly tall, the mass of the grass clippings will not be sufficient to warrant raking.
But what if you don't want to have to worry about getting the timing just right? A good way to obviate having to rake grass clippings is to mow with a mulching mower.
But making the case for using a mulching mower and leaving grass clippings on lawns goes beyond stating that no harm will be done. Leaving this residue where it falls (when done properly) can actually be good for your lawn, your health and your pocketbook. The nutrients provided by the grass clippings allow you to lower your dependence on chemical lawn fertilizers.
Research the various types of lawn movers on Consumer Search.