No. You do NOT need to set a source or destination NAT in the router. The switch will simply send packets back to the same NAC that sent the packets to the switch. That would be the router when the packets are going through the router. Remember that at the MAC layer, the next hop is all you care about. The router will look at the IP and route it.
Yes, thanks you might be correct, but it has never worked for me otherwise and with source NAT it is just always working. For reasons which are unclear to me right now, however. I was too lazy to look at the packets. Btw. my only devices on SwOS are CSS610, which are running SwOS Lite 2.14, so my experience might not apply to a css326.
BR
W