RAIN, Waterfall, and CPUIdle are all non factors in RC5. They'll just not run because a higher priority thread is requesting CPU time. Kinda like running RC5 on a NT machines. The system idle process will get out of the way of RC5 or application.
Or look at it this way, the RC5 client is like a "bizarro" Rain in a opposite world. Instead of idling the CPU it runs it full steam. When a application wants to run, the clients shuts down with no impact on system performance.
Windogg