Absolutely - if their name is in the credits, they need to have assigned their copyright and waived their moral rights or you do not have a clean chain of title. The only exception is "Special Thanks".
Here is what happens to a film if they don't.
Educate yourself: http://igg.me/at/chainoftitle/x/122306
Here is what happens to a film if they don't.
Educate yourself: http://igg.me/at/chainoftitle/x/122306