Some precise procedure...

[Jeffrey Bouquet]

Given the following procedure:

svn up /usr/src
make buildworld
make buildkernel
make installkernel
mergmaster
reboot single user
mergemaster
make installworld
pkg install compatN-i386
reboot
make check-old
make delete-old

etc [ pardon the missing stuff and out of order, this is from memory]

At which precise point either
1... Xorg fails to work
2... nvidia-driver fails to work?
Because in my experience unless a minor upgrade, it happens every time, and
I am caught unawares.. so am wanting in the summaries in UPDATING
3a... do not proceed beyond this without backups, as your video driver may not work...
and am slightly confused.
If I svn, but do not buildworld,  is nvidia-driver somehow more unusable? etc etc. 
looking at it from an entirely newbie frame of mind, because  a more authoritative
source than I may know more about the precise how and why an svn OR a buildworld
should not be attempted if one is more concerned about the driver not breaking
or being unusable 'version mismatch' upon upgrade, than the upgrade itself.

tl;dr
anyone have an expert summary?  if not, just thanks for reading, or throw a concept
at me. 
1... Xorg ceases to work and/or