Here's an awesome (long and informative) article about how intercoolers work, and how the decision of when to use intercooler water spray most effectively/efficiently is more complicated than most people assume. Their solution is an "intelligent" water spray controller monitors ambient temperature, intercooler core temperature, and fuel injector duty cycle to spray the intercooler when it needs it, and conserve water when it isn't needed.

http://www.autospeed.com/cms/article.html?&A=0527