Application U/s 91 could be filed at any stage if it is required at that stage for the court consideration to pass any orders....
Ideally, if case is false and to prove the same this application can be filed immediately saying that these documents reveals that case is false and accused/respondent may be granted appropriate relief.
No need to wait till evidence stage because sometimes some documents mat destroy due to its age... hence need to collect early otherwise what is the need for the police to collect all necessary evidence early.