Manadasytem and some others
Read the above posts re updated low fuel warning levels on the NG. easyJet are modifying their fleet so the warning comes on when one one main tank hits 453kg. I daresay other operators are the same or have been for some time.
Not all the fleet is modifyied yet so the CFPs still contain extra fuel to cater for low CNRs (below 1814) which will allow you to land above 1814kg. Once the fleet is modifyed that extra fuel will disapear as I daresay you will never find a CNR less than 907!!
You are quite right that if you get a low fuel warning you should run the QRH. Thats why easyJet allowed for extra fuel to prevent this as it was felt crews should not be dispatching into a situation were they would probably have to run a non normal checklist. That still has little direct bearing over whether you declare an emergency though (although you could if you wished to) as its generaly understood that a PAN PAN PAN with low fuel means you may land below CNR and a MAYDAY MAYDAY MAYDAY with low fuel means you will land below final reserve.
Diverting when you reach CNR is a choice not a necessity. You may have an EAT a few minutes away and the weather is fine so you would land with more fuel in the tanks than if you divert. In other circumstances diverting would be the better choice. You have to weigh up all the factors and make your decision.