If I remember correctly from my hot rod days, cold plugs burn off deposits more slowly than hot plugs, but can stand more advanced ignition timing. So cold=advanced, hot retarded. Use hotter plugs if you have trouble with plug fouling.
(Subject to the reliability of my memory)
Update:
Here's a reference that seems to agree with my memory (and a lot of additional details):
http://www.ngksparkplugs.com/techinf...000&country=US