For SELECT statement (just a reading one, no 'FOR UPDATE'), is COMMIT or ROLLBACK preferred?

Using the latest PostgreSQL, does it matter if my code does a ROLLBACK
or a COMMIT on an non-modifying SELECT statement?  My impression is
they'd be the same as nothing is changed and therefore there's nothing
to commit or rollback, but wondered if there was any difference in how
they are processed by Postgres?

Thanks,
David