They report statement balances, but not until about 3.5 weeks after the statement. To control the amount they report, just make sure your statement balance is $0, and it will eventually report, even if it takes a while to show up.
It has taken 4 months for me to kinda/sorta understand AMEX's reporting process. I've been PIF every month, but I just can't get it to work right... I've decided pay them a couple times per month and call it a day!