It needs to be called before "<html>" appears in your code. In the header section - as a rule where I use this I make it the first statement in my code. After that is used every variable that you set using session_register('var_name'); is available via $var_name.
Looking at my PHP4 book. It says to think of a registered variable as a super global declaration. Also since your variable values are stored on the server by session ID I would think you could turn globals of in your ini file.
Caution: This is my best guess. I have never tried it.