The purpose of tilling soil is to airate your topsoil. For grass seeds you would prefer to till atleast 4 to 6 inches deep break up the soil to that depth to a soft loose soil.
you should dig a spade deep careful not to get to much subsoil up then remove all debris stones and clay rake over being careful not to tread down then leave a week for the elements to break down further or wet with a hose
Prior to planting any grass seeds you need to till the entire area with a rototiller - and then you can kill any weeds with an organice killer. Then you can plant.
