My clutch gets looked at before every race day and before the main. I usually change the clutch bearings before the main and check the shoes to make sure they're not wearing. I'm running 4 carbons and they seem to hold up very well. Better even than the aluminum ones. I'll put new bearings in for the main, then over the next week when I'm doing my routine teardown to clean the whole car, I check the bearings to make sure they're OK. The slightest bit of crunch and they get tossed. They're cheap. It's worth the $2 a race to make sure I don't blow up a bearing. More than once a clutch bearing has cost me a race.
CVD's get rebuilt (cleaned and re lubed. Not new parts.) every 1-2 race days. Bearings get replaced as soon as they feel gritty. Diff's get re build and filled after 3-4 race days. I will usually do the shocks when they need it. I usually end up making an oil change to try something different before they really need to be rebuilt.