Well, per this thread
ZB Hook (needed) only global.php? it's only needed in the global.php file from what I gathered.
However since I understand oh-so-little of all this -and- I'm a bit paranoid, I also added the single line of code to my index.php; login.php and register.php files as well.(overkill? probably

)
My train of thought behind doing so was, what if someone access the register.php file directly from off-site? I wasn't sure global.php was called in that instance so I figured, better safe than sorry.
I'm sure someone more intelligent than me in how vBulletin's internals actually run could say for sure...but until then.