You aren't going to get much usable information on any port except the LAN interface of the Steelheads since all of the optimized traffic hitting the WAN port will be identified as coming from your optimized service port settings on your RB SH until it is replaced and forwarded by the SH itself.
Hope this helps!
We are using netflow on about a dozen riverbeds today and we are monitoring the lan and wan interfaces (inline config). According to the riverbed documentation netflow only tracks incoming packets to the interface so in theory if you only export one side you will only see flows in our out of the local network. I believe (90% sure) the netflow export on the wan interface knows to export the stats after it is extracted from the riverbed transport - for the most part we do not see a high percentage of riverbed transport protocol. One last thing - when you enable netflow on the steelhead, you have the option to export all traffic, optimized traffic, or passthrough traffic for each interface. We export all, but it wouldn't really make sense to export optimized traffic if all it shows it a couple big flows between the appliances.