[Users] When is RTP Proxy needed?

Hi all, We are interested in developing yet-another peer-to-peer VoIP service. Something with a simple Softphone/IM-like client that allows users to talk/chat with one another. However, and pardon our technical ignorance, we don't have 100% clear if/when we need to use something like rtpproxy or mediaproxy. Basically, we want to do all the setup of the calls and keep track of CDRs. However, we're hope that the client's actual media traffic will flow directly among the users and not have to come through our servers. For practical purposes, most users will be in domestic US with some sort of xDSL service and behind NAT. Eventually, we would like to offer voicemail services and even PSTN termination (for a fee) to these users. Can anyone shed some light as to when, if really needed, will something like rtpproxy kick in? Thanks