I like 10", but it is important to realize that every kitchen is different. What do you want to store? Dinner plates in stacks, or coffee mugs?

Thanks for this. Yes, dinner plates, glasses, decorative vases, serving dishes, etc. we would like to have 3 graduated depth shelves 8" 10" 12" or 9" 11" 13". There will also be cabinets in the kitchen so we thought for balance the shelves should approximate cabinet depth (?)

If 12" is "standard" maybe it's best not to make graduated depths? We were doing it for lighting purposes bcuz there is a space above the shelves enclosed by a beam. Light from this area should be pretty on the shelves.

I don't remember ever seeing graduated shelves directly on a wall, but is not infrequently done on freestanding furniture (hutches etc.) as it is quite practical for lighting and access.

It is true that 12" is "standard" for upper storage, but that's just because it has become the standard for cabinet box depths, NOT because it results in optimal shelf depth. The higher a shelf is, the less one can effectively use the depth and therefore it makes sense to have higher shelves be shallower. High open shelves will be easier to keep uncluttered if they are shallow.

There are lots of things that are great ideas but are infrequently done just because people don't think of it or builders don't want the slight extra work. This could be one of those ideas! I think it's a great idea and I urge you to go for it!

Example
Baker's rack http://www.dcgstores.com/48-inch-bakers-rack-graduated-shelves-gmc.html

Make sure shelves are as deep as your biggest plates. Keep them all the same depth--they'll be more versatile that way.