If I clinch the title, I certainly skip the races. Sometimes, if I know I have enough to still beat them, I'll skip Test Courses and sometimes SSR11.
And of course, there's the save-reload trick. I may try the last race of aseries as the first race so I know what I'm getting into.
The "clinched-championship" trick works for long-series races like PD Cup, for example. It's possible to win the first 7 races, and have enough "cushion" to skip the last 3 races. Why? So your car suffers no permanent loss of power!