Well, the tire changes are all up to the drivers. If the driver requests Rain Tires, he'll take them on. Today, at the Mylasian (sp?) GP, Team Ferrari's drivers chose to run with intermediate tires instead of full rain tires through the rain, and their times were posting 5 sec. faster than the others with full rain slicks.
If you saw it, Barichello made a very lengthy pit stop while the pace car was out on the track today.
This was his call, and in the end, it WORKED. A gamble, but when they work the long stop was worth it. Besides, the track was under caution so they had a few moments to spare. Overall, Ferrari has the advantage over most b/c they know what they want. Have you seen the size of the Ferrari Pit Crew??