Wool year round if really packing in the miles IN BOOTS. Coleman's Military Surplus sells a nice italian surplus (thinner) wool sock that I'll use in any weather. I'll add a liner to it in spring/fall when I typically do more miles in boots (which I rarely wear between May and September).
But when I take the trips to the mountains in summer to fly fish, or just hike a lot in general, I take a lower top boot and definitely go wool (with or without a liner... depending on how long I'll be going- longer runs get a thin poly sock liner to keep blisters down and help more moisture away from my feet through the right boot).
At home in Kansas for fishing/foraying/light work outdoors, I use a short boot (below ankle) and wool/cotton "hiking" sock (tall) when over 90 degrees. Cotton sweats in boots at ALL temps, but only if it can't breath. So at least a blend is good for me so it can wick moisture away instead of locking it in. 100% wool in HOT summer in a light hiker is fine too, as long as the boot is breathing (not a leather Gortex job) some cotton is fine when you get to those temps.
In any case... I'd hate to have to wear steel toed boots for working in Arizona during summers for sure!