I have the option of putting the shower valve body in the 2x6 wall against my fountation or in the 2x4 wall that will be between the bathroom and the bar area. Is there a recommended location? If I put it in the 2x6 wall, I can't access it later and I have to place the valve very accurately. If I put it in the 2x4 wall I will be able to access it later through the drywall or possibly an access panel. The 2x4 wall is obviously much narrower so not a lot of room for a 2x4 to mount the valve to.
Any recommendations?

Typically water pipes are not next to exterior walls, I'd say the 2x4 wall is the place for it. Should be plenty of room for the valve.

I agree, avoid the exterior wall. Your mixing valve usually has a mounting template that shows size and location of the valve hole. That hole is all you need for furture servicing.

OK, now this is in my heated basement that I'm finishing. The valve body I have is a Price Pfister and requires 2 5/8" to 3 1/2" from the front of the centerline of the inlet fittings to the face of the finished wall. I don't see how it will fit in the 2x4 wall. Am I nuts? What am I missing? I am kind of avoiding the 2x4 wall because I've already started running plumbing on the 2x6 wall side.
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I might add too that the toilet and vanity are on this same 2x6 wall.

OK Dave, you do know that you throw out that gray piece after you mount the valve, right? Back of valve to the face of the big gray surface is 3.5" which is just fine. You'll get 3.5" just from the studes let alone the backerboard and tile, it will fit. That gray thing is the template that forces you to make the right sized hole for future servicing. PS, I would have gone Moen but that's just me.

Yes, I understand that it gets removed and discarded. If I go with the smallest dimension which is 2 5/8" from the finished wall surface to the centerline of the fitting ports, that leaves only 7/8" for something to mount the valve assy to. I guess I could use 1x4 lumber.
When the builder roughed in the plumbing for the basement bathroom, they put the drain lines against the foundation side (2x6 side). Is this really a problem, and why?
I chose the Price Pfister based on the look. Lowe's didn't have any Moen shower faucets I liked. Thanks again.

If this exterior wall is in the basement there won't be the concern of freezing, that might be what they're worried about.

Buy it for looks, buy it for life. That's what Moen says so you get a lifetime warranty. Yep, I was a bit concerned about the lines frezzing. The frost line in your area is probably around 48" and your piping would be coming down from overhead so that's why I said to avoid the exterior wall.