I'm at the point on the learning curve where Av and Tv are still useful but I've taken to manual mode where I have the time to use it; It is good practice for shooting scenes where the light level changes rapidly, fireworks for example.
Since the OP mentioned motorsports, I would like to add that for motorsports particularly, shooting manual is very benificial. It's almost impossible to get the exposure correct for white/light and black/dark cars and motorcycles that follow in quick succession if you don't shoot manual!
As for highlight alert, do what seems reasonable. Car bodies in bright light will almost always have burnt-out highlights because they're shiny. Shoot and review your shots with the highlight alert on, and check out the shape of the histogram. The best shots usually have an even spread across the histogram, and therefore use the maximum dynamic range to get out all the details. Ie if the histogram is bunched up to one side you're over-or under exposing. So if you're getting an even spread, a little clipping is OK.
EDIT: For what it's worth, I've shot this image (and the entire series of the day in fact) in horrible backlight, so I had to resort to using a circular polarizing filter to cut back the reflections.