Contrary to popular belief, grass clippings are not a major contributor to thatch. Grass clippings are composed primarily of water and breakdown fairly rapidly in the soil. As long as you are following the 1/3-rule, the clippings will not accumulate on the lawn and should filter back into the lawn. If you are not mowing frequently enough, large clumps of clippings on the surface should be removed because they will decompose slowly and may smother the grass. A hint in dealing with excess clippings is to let the clumps of grass dry for an hour or two and mow them again. This will help break up the clippings into smaller pieces and distribute them more evenly over the lawn.
For additional information on grass clippings and their management see the University of Idaho’s publication: Don’t Bag It!