the failures don't usually result in kabooms afaik, just stop the weapon from working. we're talking about parts getting worn out here, not an overpressure situation.
bolts will shear lugs, and afaik, the lugs usually wind up in the receiver extension, which will usually stop the bolt from going back into battery for the next round.
bolts will also crack in half, usually at the cam pin hole, which will leave the bolt head in battery and the tail in the carrier.
http://www.hunt101.com/data/500/IMG_12541.jpg
this thread also has some interesting pictures and discussion of cracks in a colt bolt
http://www.m4carbine.net/showthread.php?t=61467
in either event, people worried about this will carry a spare bolt. magpul even makes an insert for their MIAD grip to carry a spare bolt and firing pin in the grip. the alternative, as you say is preventative maintenance by replacing parts.
what I have read is that most mil-spec bolts start developing cracks around the cam pin hole by 6000 rnds. that doesn't meant the bolt won't last 20,000 rounds... but if you were super-paranoid, 5000ish would be the point to spend $60 to replace the bolt.
it is a good idea to replace the action spring at about 10,000 unless you buy/make a gauge to show exactly when it is worn out. (there's a min/max length for the spec and so you just measure the spring and if it's too short, replace it)
extractors also need replacing occasionally, when they feel slick instead of grippy. extractor spring/inserts may be replaced when they don't have enough tension.
firing pins don't normally wear but if they chip, they will need replacement
gas rings around the bolt tail will also wear.
you should inspect each of those parts when you clean the gun. (you'll need to clean it to inspect it anyway)