We will be floating a 3/4" plywood subfloor over our slab. Based on advice received here, I had decided to go with CDX plywood. After looking at the warped, bowed stacks of plywood at HD and Lowes, I called around to a few lumber yards to ask about their 3/4" CDX. I specifically asked them about the flatness of their CDX stock and two different places told me that CDX will not lay flat to start with, even ripped into 18" sheets, and will not be as stable as a higher grade plywood once it's down. This sounds like a disaster in the making for my finished floor. They both recommended ACX for my purpose.
Is this at all consistent with what you've seen with CDX?

Thankfully, our house is small and we will only need about 30 sheets and the price difference is about $7-10/sheet. If I'm getting a better product for my application, I'm happy to pay the difference.

Thanks for your continuing help.

I could try and explain it to you but here is a forum thread, much easier for me instead of typing all that, read both pages.

http://www.nachi.org/forum/f18/cdx-subfloor-7836/

While I don't know everything about subfloors for installing hardwood I do have one recommendation. But this is also a question for the more experienced hardwood installers because I think its a good ideal and do not see how it could hurt anything. So my recommendation/suggestion is if it is appropriate to sand the seems on the underlayment if its 3/4 or thicker. I have also heard of using a nail as a spacer between the sheets of underlayment and this is a cleaner method but not usable on remodels.

i have used both not by choice but what has been bought for the project, personally never cdx usually because its a yellow pine which has alot of movement, i alway prefer a fir,(stable) but what ever you use it should be t&g. that way you will not have a seam problem when the slab is not level.

if I had my choice it would be two layers of 7/16 or 3/8 ripped down in 8 x 2 panels laid perpendicular to each other staple together and nail to that of course it should be over felt plastic felt sandwich.

Thanks for all of your replies.

I will try to find the flattest plywood I can that is not pine.