When do I replace chain rings
 

I have a bent large (53T) chain ring, Dura Ace, so need to replace that, and am also going to replace the chain and cassette as well, as these have never been replaced in 3 years (about 12,000 km). My question is, do I go the whole way and replace the small (39T) chain ring as well? It looks OK, with no obvious signs of signficiant wear etc., but if everthing else is new is it better to change small chain ring - if I don't will it wear out the new chain/cassette faster? Cheers :)

It won't wear the chain/cassette at all because it is aluminum (soft) and the chain/cassette are steel (hard).

The only real question is the amount of wear on the small ring; if it's worn you replace it, if not you clean it.

BTW, a chainring that is bent (not severely) can often be straightened.

THanks for your reply. Regarding staightening bent chain rings, I tried this with pliers and it improved a bit, but chain still came off, then I took chain ring off bike and hit with a big hammer and this straightened it some more - it now looks perfect to the eye but chain still comes off when sprinting in big ring. Do you have any other suggestions or am I right to just get a new chain ring? THanks :D

A good guide to ascertain chain ring wear is to look at the teeth, if they are not symetrical (ie; looking like a shark's fin) then most likely they are ready for replacement.

He said you can often straighten chainrings, he didn't say go grab some pliers or a big hammer and start whacking away and see what happens.... your mechanic at your LBS (if experienced) will have a little specialty slotted chainring straightening wrench that he can use to tweak the offending ring back to some semblance of straightness. It takes like 2 minutes.