Hi,
Refer : http://www.cisco.com/c/en/us/td/docs/security/firesight/541/user-guide/FireSIGHT-System-UserGuide-v5401/Managing-Devices.html#pgfId-7283033
The above link explains deployment of Firesight. Now it totally depends on what network you are deploying it . If you are going to manage the devives in same the subnet then you can deploy that in lan. Generally customers who has mulltiple devices in the network being managed by Firesight . They deploy that in WAN . If you have your FMC in WAN and we have any NAT device in between the only difference in registration is that you have to add the NAT id.
Regards,
Aastha bhardwaj
Rate if that helps!!!