The steering on my 2011 E350 sport had no on center feel and required constant tiny adjustments to keep it in lane. I had the alignment checked and it was spot on front and rear.
Reading many posts on various sites I found that low profile tires are notorious for following every groove,rut.ridge and tar snake on the highway. The cure is increased tire pressure. On my car the gas cap door says 30 psi front and 35 psi rear for normal loads. The drivers door jam says 33 front and 42 rear for max loads. I tried 33/35 and found no improvement. I upped it to 35/36 and the improvement was amazing. I now have good on center feel and the car tracks straight down the highway with no need to make constant adjustments. Just a few pounds pressure made a huge difference.
If your steering is "nervous" or you have to make constant tiny inputs to keep it in lane try upping your tire pressure, especially in the front. In my case going from 33 to 35 psi produced a huge improvement.