The amount of grains and vegetables in a healthy homemade diets should be relatively small. Carnivores animal such as cats would only consume plant matter found in their preys intestinal tract. Dogs can be more omnivorous and can eat more plant based foods.
If you choose to feed a raw or cooked homemade diet it is best follow a recipe for a nutritionally complete diet. If you are guessing, you are taking the risk of your pet having a nutritional deficiency. Once possible deficiency is a lack of calcium for pets eating homemade diet without added bone or calcium.
If you are interested in making a healthy homemade diet for you cat Catinfo.org is a great resource with tons of detailed information on preparing a home made diet for your pet.
Another one of our favorite resources is Thomason Feed in Cheshire. All of their staff is extremely knowledgable and they can help you pick healthy ingredients as well as pre made nutritionally complete diet for your pet. Even if you plan to start preparing a homemade diet it is always convenient to have pre made food on hand for when you may be short on time. They carry a great variety of dry, canned, freeze dried, and raw foods. Please stop by their store or visit their website for more information.
Here are some additional links of interesting pet food information
Dr. Beckers Real Food For Health Dogs and Cats Cookbook