I have spoken to a few bartenders (locally and in the UK) and they have all advised that single malt should have a touch of water added to bring out the flavour and aroma. I have found this to be quite true (as per my own personal tastes). However I still have friends that prefer it straight or with ice. To each their own.

I don't know if the same would apply to a blend or an Irish whisky though.

The only whisky I would ever add any kind of "soda" to is Canadian rye whiskey. I practically grew up making "Manitoba Martinis" (rye and 7-up) for my grandmother. I still have one if I am at the curling rink watching a game.