Not sure about 'ildct', but IMO '-alternate_scan 1' should be set for every interlaced encode.
Have a look here (I have no idea how old this is...)
http://en.wikibooks.org/wiki/FFMPEG_...ags_Flags#ilme
My personal preference would be to also set 'ildct' for all interlaced encodes...
Chees
manolito