the reason for boiling tap water is the same as the reason for sterilising of bottles - its all rather unnecessary these days in the UK. the original point of boiling tap water and sterilising is because our water wasn't clean - now it is, negating the need for boiling the water and sterilising bottles (as long as they're washed at over 65 degrees and thoroughly clean).
having said that, even though i know there's actually no need, i boil my water and sterilise my bottles - i'll probably stop when connor starts chewing on things and sticking other stuff in his mouth though.