It is up to you really. I still dilute juice for myself! I find it too sweet and thick and I like just a hint of flavour. I am a big water fan, so no surprise DS is. In many instances "juice" is not juice but from concentrate that has hidden or added sugars. The main concern at this age is tooth decay and filling up on empty calories. Pure juice is good though if you have a tough time getting fruit into your LO as it is a good source of minerals and vits. Everything in moderation I suppose.
HTH,
Aisling x