So, here's a quick question. Are "warmcards" worth it for documentaries shot on the HV20, especially for sit-down interviews? Do you still have to do color correction in post or can footage pre-balanced work as-is?

You *can* color correct in post. If you have a budget for it, they are always nice to have. Saves some work later. If all your temps look good, why do the extra work in post? You have to do some amount of white balance without them anyway, just to get in the ballpark.

imo anyway

Forum search is your friend:
http://www.dvinfo.net/conf/showthread.php?t=5434

If you have a properly calibrated monitor, and you're willing to accept a particular look, then you should try to get that look before it goes through compression and hits the tape.

Otherwise, if done in post, you'd have to apply the effect or look to the entire piece, and thus increasing rendering times and risk recompression and further destruction to your media.

Some caveats or situtations to consider is when shooting comercial products, and respecting colors of their branded materials (i.e. matching a particular pantone color), then warming/color shifting should be avoided. (You still may need to use filters to correct color).

The other fact is that in aquisiting, any lens filtering is a global shift in the scene, and if you want localized correction, that will have to be done in software, (with masks and keys), and you should shoot as neutral as possible.

Printing your own warm, or minus green cards is ok so long as you know that your printer is setup and calibrated as well. I've done this already, however, for the cost, it's cheaper and convenient to buy a set of cards and ensure consistency from shoot to shoot.

In regards to your HV20, test, test, test. Only you can judge if it's suitable for your situation.

I find using the warm card 1 (and 2) for white balance to be a definite improvement over the HV20's WB, or auto White Balance. It takes a flat desaturated look and really 'warms up' skin tone quite nicely, saving extra color correction in post. There are other work-around's, but I find the warm cards to be convenient and worth the purchase price.