In my experience, most proxyservers tend to nat the client ip to an common egress address, most often the address of the outgoing proxyinterface and simply apply pat to handle multible sessions.
Weather or not you need to configure src.nat on egress traffic, would depend entirely on your setup. In some cases, src.nat is needed to avoid the possibility of asynchronous routing.
Don't know if this answers your question, but more accuracy would require some more insight into your proxy/ace setup.
hth
/Ulrich
