How many clamps?

I've got the basic boxes of two of my bookcases built, stained and finished (inside surfaces only; the outsides of these won't show).
Looking forward to the face frames, I wondered how far apart to put the clamps when I glue them on. I intend to use pocket screws around the perimeter (again, because the outsides of these "built-in" units won't show), but I think the frame pieces that edge the front of each shelf (they are all fixed) will simply be glued and clamped. The frames will be screwed and possibly glued together as a unit beforehand.
So I did a little searching online and found that some people recommend festooning practically every square inch of any mating pieces with clamps, even when there doesn't seem to be any obvious stress involved. Can this really be necessary, or is it simply to prove the old adage?
Here's how I glued up the boxes:
http://www.flickr.com/photos/gdguarino/8442008425/in/set-72157632376881493/lightbox /
I didn't worry too much that I only clamped the dado joints at the top (actually the back) and bottom (actually the front) of the units. Should I have? The quick-grip clamps at either end of the units were just to hold the sides on while I applied the other clamps, by the way. Could there be enough flex in 12" of plywood that there would be insufficient pressure in the middle? When I glue up the next two units, should I put, say, a 12" piece of 2x3 on the outside of the box opposite each dado joint to help spread the pressure to the middle of the plywood sides?
I could of course have simply used screws, as the sides of these units won't show. But one side of each of the next two units will be exposed, and I figured I might as well get in some practice.
Thin as my experience is, I have glued together some boxes before, and my intuition tells me these aren't coming apart, especially once I put the backs and the face frames on. Is that wrong?
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On 2/7/2013 2:24 PM, Greg Guarino wrote:

http://www.flickr.com/photos/gdguarino/8442008425/in/set-72157632376881493/lightbox /
Use enough clamps to insure even contact. If you have even contact you really don't even need clamps but clamps will straighten out bowed wood and will help prevent pieces from slipping.
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On 2/7/2013 3:24 PM, Greg Guarino wrote:

http://www.flickr.com/photos/gdguarino/8442008425/in/set-72157632376881493/lightbox/

Dude, nice work. even though you won't see the other side, put a light coat of finish on it. It will help to seal it so that both sides absorb moisture at the same rate. You don't have to make it pretty, just purposeful.
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Jeff

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On 2/7/13 5:22 PM, woodchucker wrote:

With shelves glued every foot, I don't see how that plywood could ever warp.
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-MIKE-

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On 2/7/2013 6:50 PM, -MIKE- wrote:

true, but it will put stress on the case.. and it won't hurt.
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On 2/7/13 5:58 PM, woodchucker wrote:

A lot of completely necessary things won't hurt. I'd love the see a measurement of the "stress" involved. I'm guessing any books sitting on the shelves would put a stress on the case at least 1000x of what would result from of any atmospheric humidity on the unfinished side of 3/4" cabinet plywood in that situation. Maybe 10,000x.
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-MIKE-

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<Lot of stuff snipped --sorry>

My Standard clamping system: 48 clamps. 3" apart. :) <sorry>
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