RTS/CTS in ns2 can be turned on/off by setting the Mac/802_11 set RTSThreshold_ parameter to a certain value. Is the packet size bigger than this value, RTS/CTS will be used, otherwise it won’t. The default threshold is 3000 bytes. Recent 802.11b cards apparently do not use RTS/CTS as this degrades the performance. This is not necessarily the case in ns2: some papers report a performance increase, others report a decrease.
In my simulation there was no difference at all. Trace files with and without RTS/CTS are 100% identical. This may be broken in my ns2 because of the Rice Network Group Multi Rate patch.