My theory is to till as deep as you can. The better the quality of the soil, the better the plant.

As deep as your tiller will go won't be too deep.

You don't have to til real deep, as long as your soil is good to a dept of about 16 inches. Sometimes you can even skip tilling and just plant. But if your soil is clay, for example, and you have just 8" or so establishe top soil, Then you have to dig a good shovel depth, turning it over, raking, to get everything blended. At such time, if your shovel is hitting hard bottom or when turned shows heavy clay, maybe you have to add and mix some more compost/top soil/leaves..

Potatoes, beets, and turnips will do fine with 6 inches of loosened soil. Carrots would like 12 inches, and parsnips 12 to 18 inches for best results.

Who has a homeowner type rototiller that can go down more than four or five inches? I have a Troy-Bilt Horse rototiller with an 8 HP engine, which in their advertising they claimed could go down 8 inches, but in my clay soil it can really only go down about four or five inches, and that's after repeated passes. It looks like you might be down 8 inches, but that's just because it fluffed up the soil on top. If you scrape off the fluffed up soil with a shovel to see how far you really got down its only about four or five inches max. Maybe one of the big commercial rototillers can get down deeper, or maybe in sandy soil.