Not necessarily. I think it's just "better" to install perpendicular but not "necessary". Or maybe depending on age of house or subfloor material it might become necessary.
http://ca.answers.yahoo.com/question...2080410AAR7alg
The only point I really wanted to make is that it's better to check/confirm before acting on assumptions, particularly when it's easy enough to get a definitive idea where the joists run.