Do I run Mkfs (Newfs) on the swap partition?
No. The “Build Devices” menu in the Installer utility will create /etc/fstab for you with the appropriate entry for a swap partition. When you boot into NetBSD, /etc/rc will execute the swapctl command and mount your swap partition. Please note that versions 1.1a through 1.1c of the Installer create a possibly incorrect version of /etc/fstab putting swap on sd0b. The solution is to obtain Installer 1.1d or later.