I've never seen anything shrink like 5/4 wood. Install it as tight as you can and in a year, the gaps between boards can be from half an inch to a full inch. If you don't use it in a day or two after buying it, the warping will amaze you!!!!
I have never had 5/4 shrink that much, usually just 1/4" or so. I wonder if the difference is due more to differences in local weather or to the nature of PT available in different markets?
I have had unused boards turn into bananas, and others that remain straight.
Sorry if I wasn't clear. My comment is on modern day treated lumber that has the new formula and treatment methods that force a huge amount of moisture into the boards compared to the previous method. This massive amount of moisture remains in the boards for months and causes all of the issues that we have to deal with in modern day pressure treated wood. 5/4 lumber is probably the most affected by this, but 4x4 posts are also junk for anything taller then a hand rail, and even those short lengths can twist on you a month after you install them. What you might have been happy with ten years ago is not the same thing that is sold at the stores today.