Great question and one I'm struggling w/myself.
I posed the same question to my Vet. In our discussion when Curry was first diagnosed I asked if the canned wet food could cause more dental problems as I had heard the dry was better for their teeth. He said not necessary so anymore. In the past, most of the research was done w/dogs who are easier to study as a kitty's system is much more difficult to assess. They had found that cats chew differently than do dogs so the canned food isn't worse in terms of dental care.
I did ask him about "greenies". They have an enzyme in them that helps reduce tartar buildup. Of course, the greenies package says to give more, but I never did. I would only give one in the AM and one in the PM before Curry's dx after her Flovent tx for her asthma. He said that small amount would be OK for her and not change her BG much. However, since I'm so new to this and Curry is slowly adjusting to the canned food and insulin, I'm still leery of giving them to her. I might give her one a day w/her food every other day. As her BG levels become a little more regular, I will revisit my old practice of 1 greenie per pre-shot meal and see if I notice any difference w/her BG levels.