So this is a two part question, the second of which involved the update to Raspbian Stretch.
1. I have used the cmdline.txt option ip=xxx.xxx.xxx.xxx for a while now to set a static IP on several Pis. It works just fine on my Pi B, Pi 2, and Pi 3, which are all connected via Ethernet. I have a Pi Zero W that I am trying to set up over wireless. I put the wpa_supplicant.conf in the boot partition to get it to copy over, and that works no problem, but the Pi gets an IP address from the DHCP server. Any idea why the cmdline.txt option doesn't work for Wi-Fi? Once I set up a static IP for the wlan0 interface in /etc/network/interfaces it works. Which brings me to my second question.
2. On Stretch, the /etc/network/interfaces file doesn't have the normal interface options (auto wlan0 inet manual, etc. whatever else is in there). This is where I normally set my static IP for my wireless connection. Has this changed somehow? If so, where do I set my static IP now?