VoIP with or without firewall???

Hello:

In my company we are implementing VoIP. Some customers are installed without firewall, and are operating well, but we are concern about security and reliability, so thats why we want to use PIX or Firewall between Cisco AS5300 and router devices.

The question is, what is most preferable, use VoIP with or without firewalls? I understand that this security device add delays in the comunications also, but what is the most common practice?

Thanks.