i had filled an application for cancellation of gstin for one of my client on 06-03-2018 which shows status "Pending for Processing", now i want to ask that is it mandatory to file nil return even after applying for cancellation?
Cancellation of registration is not your right unless the law deems it so on expiry of prescribed period (if any). The proper officer has to examine your liabilities of taxes etc. as per law before cancellation is allowed. Therefore, it would be appropriate to file the returns till cancellation is formally done.