If you're using PXE to make the servers boot, then it makes sense to use DHCP.
You can assign static IPs within DHCP - but be careful, if you're using iSCSI root devices without a HBA (iPXE and so on) you can't change the IP after boot time because it will disconnect your root iSCSI device.
Windows (when netbooting over iSCSI) will get two IP's at boot, one from iBFT and one from DHCP Client Service. Disable DHCP Client Service on netbooted Windows devices. Linux is guilty of this as well - remove NetworkManager and similar software from the PXE image.