No the indexing service does not need to be enabled.
IFilter components are completely independent of the indexing service, most filters would work on a machine without any indexing service at all. An IFilter is just a standard (a COM API) for searching documents that the indexing service happens to use.
Technical information on the IFilter API can be found here:
http://msdn2.microsoft.com/en-us/library/ms691105.aspx