For SELECT statement (just a reading one, no 'FOR UPDATE'), is COMMITor 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