This site discusses dog food ingredients in depth.
http://www.mordanna.com/dogfood/index.php?page=identify
This is the section discussing grains:
While grains can be a beneficial addition to a dog food, some are more and some less digestible. Rice is the most digestible, grain sorghum, millet and wheat are further down on the list, corn ranks last. Grains like quinoa, kamut and amararanth are more expensive and not as well known but provide a wealth of nutrients. Quality is also important, since grains deemed unfit for human consumption are often contaminated with mold. If in doubt, ask the manufacturer what types of grain they use - the more trustworthy companies usually make such information freely available on their websites.
Whole, unprocessed grains (also listed as "ground" or in "meal" form) are important because they still retain all the nutrients in the bran layer, the original fat content and the endosperm. Grain fragments and flours are incomplete and have already lost much of their nutritional value during processing and storage. Some manufacturers include one fragment in order to lower product cost, but be wary of products that consist mainly of fragments and flours.