...the newly tilled dirt? I am tilling and sodding an area around the existing grass and just for uniformity and to make sure the grasses are the same, I was going to sod over the existing grass area. The existing grass is growing fairly well but is uneven and a bit clumpy. I want to till it and then roll the whole area before laying the sod so I have a nice smooth surface.

That's how I did it, however, I also raked it throughly to remove all the old grass clumps, raked it again to level the ground and then planted seed and within a couple of weeks I had a lawn. Sod would have been even better, but this was just a small area. Sounds like a solid idea to me.
Good Luck