You need to rake them or the grass underneath may be smothered. Be sure the mower blade is sharp and cuts, not tears, your grass. A sharp blade will also help chop the clippings into smaller pieces as you mow over the lawn time after time. A mulching mower works even better, because it is designed to chop up clippings very fine and then deposits them down in the turf canopy. Despite rumors to the contrary, clippings do not promote thatch build up.
Clippings break down quickly, often in a matter of a few weeks.
In most cases, thatch increases as mowing height increases.