FreeBSD is currently in the process of moving everything from X11BASE (/usr/X11R6/) to LOCALBASE (/usr/local/). Which is the reason I've set my X11BASE to /usr/local in /etc/make.conf so I can test if everything works from /usr/local instead of /usr/X11R6, I don't even have a /usr/X11R6 directory anymore.
I'm working on a project where several systems with multiple nvidia cards each will be put into production with the drivers and software "frozen" at the versions with which everything was tested, and anytime within the next few years, we may need to be able to regenerate a system from scratch with a properly working 1.0-9746 nvidia driver.
What happens if one of the tested versions turns out to be vulnerably to some security related bug? There are already 2 security advisories affecting 6.2-RELEASE. Some goes for the ports..
On my systems I usually create my own packages, I use one system for building them and install the resulting packages on everything else. These packages can be stored and used later on when reinstalling everything. I mainly use portupgrade for this. You can portinstall -Rp nvidia-driver and it will build/install the driver and create a package tarball in /usr/ports/packages/, it will also build/install/package the dependencies. You would need to remark the NO_PACKAGE in the make file though. Seeing you'll probably be using the same kernel this wouldn't be a problem.
You may also want to look into doing a "make release" in /usr/src. That will create your own release CD at any given point in time (patches, kernel/os version). If setup properly it can also build several ports you've chosen and can be used to create your own fully automated installation CD which will also include/install all the packages you need.