PCI DSS Compliance for Level 1 Merchants and Service Providers is mandatory. In short, if you are a Merchant or Service Provider and have been called upon to become Payment Card Industry Data Security Standards (PCI DSS) compliant, then an on-site assessment by a Qualified Security Assessor (QSA) is what you will need.
A QSA is simply an individual who has gone through the licensing to become an expert in PCI DSS compliance. This is somebody who has been awarded the designation by the Payment Card Industry Security Standards Council, known as the PCI SSC.
For more information about PCI DSS compliance and in hiring a QSA for all your Level 1 needs, visit the official PCI DSS Resource Guide.
And lastly, MasterCard has now strengthened their requirements to make Level 2 merchants also undertake an on-site PCI DSS assessment.