I had always heard that dry food was better for my dog and cat because it helps keep their teeth clean, I even remember TV commercials about dry treats that would clean the dog’s teeth.
In the question Is there anything beyond preference that dictates whether dry food should be wet down? comments suggest that dry food is not really helpful. Is the whole "dry food = better teeth" thing a myth or what?
Trying to find some peer-reviewed article or book about the subject, I found "The Manual of Veterinary Dietetics," from Saunders / Elsevier.
The conclusion is that Soft vs Hard foods don't have any clear winner. Many studies were made or reviewed, and the authors found it hard to compare the conclusions because each study involved different animals, with different methods of analysis.
Those authors' conclusion was that animals of the same breed may have more tendency on developing dental problems since their anatomy is different, for example, having more or less space among the teeth.
Some special food (textured dry food), on the other hand, could be beneficial in the plaque removing process, and flat rawhide chews of small sizes could help too.
Source:
MANUAL OF VETERINARY DIETETICS 0-7216-0123-5 Copyright 2004, Elsevier (USA). All rights reserved. Authors: Tony Buffington, Cheryl Holloway and Sarah Abood.
It depends - if the dry food is going down without being chewed (which is most of the time in my experience), then it's not going to do any more for a dog or cat's teeth than anything else.
Vets I've spoken to have also said that dry food isn't necessarily going to be much help with keeping teeth clean - they recommend uncooked chicken bones for cats. For dogs, there are a range of chews you can get them, or you can simply give them a chew toy to destroy (Or convince both dog and cat that they like you cleaning their teeth).
