Rototilling alone will not produce a good lawn. Clay will soon re-compact. In southern Oregon, we have some very heavy, sticky when wet, clay. When I was installing lawns for a living, I always incorporated 1 yard of granite, and 1 yard or more of mulch, per 100 s. f..
The sharp granite helps to penetrate, and break up the clay. The mulch puts organic matter into the soil. This makes for a good root zone. You need to till at least 6 - 8 inches. Don't worry about removing any rocks smaller than softball size. Rocks help drainage, and one the lawn in established, you will have forgotten they were ever there.
As you may know, clay can only be worked when the moisture content is PERFECT. To damp, and you will have balls of clay coated with granite, and mulch. To dry, and you will beat yourself, your tiller, and your tractor.