Do I need DMARC if my domain does not send email?

Yes, and it is easier than for a domain that does send. An unused domain is an attractive one to forge:

• Publish a null SPF record - declaring that no server is authorised to send for the domain.
• Publish DMARC at p=reject - with no legitimate mail to break, you can enforce immediately.
• Cover the parked domains too - the brand-protection registrations nobody thinks about.

This is the rare security change with no rollout risk: there is no legitimate mail to disrupt.