A common misconception of many people are the terms "Hot" spark plug, and "Cold" spark plug. The temperature rating of the plug refers to the running temperature of the physical spark plug - i.e. a "Hot" plug will retain more of the combustion heat in the plug itself, meaning not transfer the heat to the engine itself. This is why a "Hot" plug is needed for slower city type driving so the carbon deposits will be burned off the plug, and clean firing will still be possible even when combustion temps are low (idling, stop & go etc.). At the same time a "Hot" plug used in a highly modified engine that is driven hard consistently will simply retain too much of the extra combustion temperature and burn itself up (usually cracking the porcelain, and potentially doing major engine damage).
A spark plug that is too "Cold" for the engine/application will start harder, foul much easier, and generally be a pain to work with.
For engines that are not significantly modified for racing, we have had very good luck just running the stock plugs. For street ported engines driven hard on the street, the most we have had to do is move one or two heat ranges colder. This lack of change is because the engine is still driven at normal speeds and loads for 99% of the time.
Racing engines, for the most part, are above 7000rpm 100% of the time - these are the ones that require the colder plugs. Quite often a set of "hotter" plugs must be used to get the engine started and warmed-up, because the race plugs will not fire consistently below 4000rpm.
Spark plug choice, for any given engine, can then be stated as "Hot" enough to fire consistently at the lowest RPM and load normally needed (without fouling), and not overheat (and burn-up) at the highest RPM/load for the given application. This translates to : The "Hotter" the engine (turbo, modifications, load, usage, etc.) the "Colder" the spark plug needed.