What PP said!
Also it depends on the quality of your tap water, UK is generally good but we do get times when there have been cases bugs in drinking water so I have always played safe and boiled the water. My DS has some reflux formula until he was nearly 2 and DD still gets some at bedtime and it really isn't that big a deal to put a bit of boiloed water into a bottle.
The recs in the UK are to follow the directions on the formula packaging for as long as you are giving it (and that includes sterilising bottles as milk breeds bugs fast) and if giving water to babies it should be boiled and cooled up to 6 months.
Laura