If you accept credit cards, the answer is: yes.
As of July 2010, merchants (including restaurant operators) are required to use only PCI PA-DSS validated point of sale and payment applications.
Financial institutions enforce the requirement for an annual PCI security self-assessment and quarterly network scans, and can levy fines for non-compliance. If your POS system is non-compliant, you will automatically fail your PCI assessment, and could lose the ability to accept credit cards.