I would never ever install a codec pack, but that's just me, there have been too many that have bad codecs in them.
If you're only viewing, not encoding, there are just two that I would install, divx3 (there's an audio codec in there that you can't find elsewhere) - and ffdshow. Those two cover 99.99% of all that's out there (excluding quicktime & real player, of course) - and you get much better performance with ffdshow than the official divx codecs (well, unless the newest divx codecs have changed that, I don't know).