I would use stainless screws. Does everyone know they changed pressure treated lumber and you need to use stainless or double dipped galvinized. I always use stainless screws outside, not a big deal cost wise.
I know the thread is old, but...
We've used stainless steel screws. First box we bought was the last. Horrible. Bent easy, so had to be much more careful using them.
We've used GripRite's Primeguard exterior screws, since we're not using them on "docks, piers, or within five miles of coast line". I'm sure there are some applications where stainless steel would be preferable, but the aggrevation of the "softer" screw which bends easily is just not worth the hassle.
Note: it was using LONG screws to marry boards together that we first tried these, so you may not have the bending issue with shorter ones attaching fasteners.
It is my understanding that the "new" version of treated lumber is much more damaging to fasteners. I was told that even though it is arsenic-free, you should still wear a mask and not put sawdust in your garden. More extreme pH is hard on the lungs and tempermental plants may not like the pH changing -- and you don't want to kill beneficial insects with it in your plants.