The relevant rule is clear:

87N RESERVATIONS FOR THE FIRST INTERNATIONAL FLIGHT, AND
88N ALL PRECEDING FLIGHTS, ARE REQUIRED AS FOLLOWS:
89N .
90N ------------------------------------------------------
91N I ORIGIN IN AREA 1 I ORIGIN IN AREA 2/3
92N ------------------------------------------------------
93N I 7 DAYS I ANYTIME PRIOR TO I
94N I I DEPARTURE OF THE FIRST I
95N I I FLIGHT. I
96N ------------------------------------------------------
97N . * ALL OTHER FLIGHTS MAY BE OPEN.

But one complicating factor is:
2N SEASON
3N ------
...
10N . * ECONOMY CLASS - DETERMINED BY THE COUNTRY OF
11N . TICKET ORIGIN.
12N THE DATE OF THE FIRST INTERCONTINENTAL FLIGHT
13N DETERMINES THE APPLICABLE SEASONAL FARE FROM THE
14N COUNTRY OF ORIGIN FOR THE ENTIRE JOURNEY.

So in Business or First it really doesn't make a difference, but if you are travelling in Economy then I can see that there is in fact a conflict between line 97 above and the requirement in line 12. I've not noticed this before since I don't do economy long haul, but I can see it might be a problem.