Do I still need to submit a monthly CIS return if I haven't paid any subcontractors?

Yes — if you're registered as a CIS contractor you must submit a monthly return by the 19th of each month even if you made no payments to subcontractors that period, in which case you file a 'nil' return. If you know in advance you won't be paying subcontractors for a while, you can tell HMRC you're temporarily inactive to avoid needing nil returns each month. Missing the monthly deadline, including for nil returns, can trigger automatic penalties, so it's worth setting a recurring reminder or letting your accountant handle the filing.