[vlc] Newbie question: When to use deinterlacing ?

[Ton]

(I posted this question also in the VideoLAN forum. I hope this is not a 
problem or not-done)

Recently I started using VLC as the default player on a pc that I 
assembled specially for playing all kinds of video files on my tv, and I 
must say that I am impressed by the performance and the ease of use. 
However one thing is not completely clear to me and that is when I have 
to use deinterlace. I must admit that I am rather non-technical and a noob.
My tv is a HD-Ready one that can handle 1080i or 720p and I set the 
output of my video card (connected through a DVI to HDMI adapter to the 
HDMI input of my tv) to 720p.
I thought to have understood that deinterlacing is not needed when 
playing dvd files and that it only should be used for tv material. But 
with some dvd files (I mean files on the harddisk and not dvd discs), 
for instance a concert movie I watched yesterday, I get nasty scanning 
lines with fast moving images. When I set VLC to "Deinterlacing: Blend", 
they were gone.
Would it be a good way to analyze a file first with for example 
MediaInfo and when the info says "Interlacement Top Field First" then to 
use Deinterlace ? Or are there other/better ways to judge what to use ?
TIA
Ton