You don't have to sterilise anything ever IF you wash items thoroughly, with the water at very high temperature. We are told to sterilise as a matter of tradition rather than through scientific necessity - its worth noting that they don't tend to sterilise bottles in the USA.
I stopped some time ago as, like Lil Miss, Connor had anything and everything in his mouth.
Sterilising used to be done when our water supply wasn't clean (major reason why the encouragement of formula feeding by formula companies, in 3rd world countries, has led to so many baby deaths). Our water supply is now clean therefore there is no need to sterilise.
Milk bacteria in improperly washed bottles can lead to gastroenteritis, although the majority of gastroenteritis cases are viral and not bacterial. And over-sterilisation has been linked to the increase in childhood leukeamia.