I've built 3 decks with cedar - the first two, I left a small gap between the board (I built a small jig to space them and align the screws on each board). The third deck I butted them together. The ones with the spaces now have clearly larger gaps from the boards shrinking. The one that I butted together now has reasonable spaces for water drainage, etc. This was with cedar, not PT, but my mind is made up: Butt them together and let them make their own spaces!

I suppose the success of this depends highly on how wet the wood is when you use it.