If you have multiple machines that you want to track the same source tree, then having all of them download sources and rebuild everything seems like a waste of resources: disk space, network bandwidth, and CPU cycles. It is, and the solution is to have one machine do most of the work, while the rest of the machines mount that work via NFS. This section outlines a method of doing so.
First, identify a set of machines that is going to run the same set of binaries, which we will call a build set. Each machine can have a custom kernel, but they will be running the same userland binaries. From that set, choose a machine to be the build machine. It is going to be the machine that the world and kernel are built on. Ideally, it should be a fast machine that has sufficient spare CPU to run make buildworld and make buildkernel. You will also want to choose a machine to be the test machine, which will test software updates before they are put into production. This must be a machine that you can afford to have down for an extended period of time. It can be the build machine, but need not be.
All the machines in this build set need to mount /usr/obj and /usr/src from the same machine, and at the same point. Ideally, those are on two different drives on the build machine, but they can be NFS mounted on that machine as well. If you have multiple build sets, /usr/src should be on one build machine, and NFS mounted on the rest.
Finally make sure that /etc/make.conf and /etc/src.conf on all the machines in the build set agrees with the build machine. That means that the build machine must build all the parts of the base system that any machine in the build set is going to install. Also, each build machine should have its kernel name set with KERNCONF in /etc/make.conf, and the build machine should list them all in KERNCONF, listing its own kernel first. The build machine must have the kernel configuration files for each machine in /usr/src/sys/arch/conf if it is going to build their kernels.
Now that all that is done, you are ready to build everything. Build the kernel and world as described in Section 188.8.131.52 on the build machine, but do not install anything. After the build has finished, go to the test machine, and install the kernel you just built. If this machine mounts /usr/src and /usr/obj via NFS, when you reboot to single user you will need to enable the network and mount them. The easiest way to do this is to boot to multi-user, then run shutdown now to go to single user mode. Once there, you can install the new kernel and world and run mergemaster just as you normally would. When done, reboot to return to normal multi-user operations for this machine.
After you are certain that everything on the test machine is working properly, use the same procedure to install the new software on each of the other machines in the build set.
The same ideas can be used for the ports tree. The first critical step is mounting /usr/ports from the same machine to all the machines in the build set. You can then set up /etc/make.conf properly to share distfiles. You should set DISTDIR to a common shared directory that is writable by whichever user root is mapped to by your NFS mounts. Each machine should set WRKDIRPREFIX to a local build directory. Finally, if you are going to be building and distributing packages, you should set PACKAGES to a directory similar to DISTDIR.