I always see sweets precubed for cooking anyhow.
I'd probably err on the safe side and store them in water, but it kind of depends on what you're making with them. Is it something where a little oxidation would matter, or no?
I agree with Merrill, and go for the water treatment. They don't "purple" quite as badly as white potatoes do, but "purple" they do as they oxidize (they contain less water than white potatoes, so they oxidize more slowly). I'd vote for avoiding last-minute surprises or replacements.
I'm making a gratin -- this one: http://food52.com/recipes...
A little oxidization won't hurt, I suppose. I also might leave them uncut but peeled.