Just so you are aware there is an IEV 5.2.
IEV 5.2 can receive events from 5.x sensors as well as 6.x sensors.
Though with 6.x sensors some of the fields in 6.x will not be seen in IEV 5.2.
http://www.cisco.com/cgi-bin/tablebuild.pl/ips-ev
So you would only need CSM if you were wanting centralized configuration instead of per sensor configuration with IDM.