There's some truth to the holding in hits "myth;" you DO get more THC from holding it in for a long time. However, you get very little extra THC and lots of extra tar and other bad stuff.

The way I understand it, the majority of the THC your body absorbs happens almost instantly, when the smoke first hits your alvioli (sp?). After that, holding it in will cause you to continue to absorb a portion of what's left. The nasty stuff (tar et al) get deposited much more slowly. So, if you want the absolute maximum possible THC from your hit, hold it in. If you want the absolute minimum possible tar, breathe out immediately. Most people settle for a happy medium around 5-8 seconds.

It's been a while since I've seen numbers, but I think you get about 20% more THC holding it in 5s than 1s, 25%-ish more if you hold in for 8. After that, the added THC is almost definitely not worth it.