On Pressure Treated wood, this applies to 2x6's more than that 5/4 wood. There was the idea that its good to use a 16D nail as your spacer when butting up decking. When dried, especially in hot climates like mine, that will yield about alittle less than 1/2 inch space.
I feel and many others that I have discussed this with, this is too much. Most now will butt up tight and then you will get about a 1/4 inch after drying. This is acceptable.
But, when building a deck in the fall or winter, I tell the customers that they will not get that spacing until two months into the summer. PT wood will not dry or shrink during most fall, winter, spring seasons.
Now 5/4 will shrink even more, so I would definatelly butt that material.
As you probably know its ideal to place decking on the floor joists with growth rings in a downward cup design. This will reduce cupping and ponding of water within a board and also splitting and other defects. You can not always accomplish this task as the good side may be full of knots, but only a rule of thumb.