If I peeled sweet potatoes the day ahead, do I have to store in water or is a ziploc OK?

I'm making a gratin -- this one: http://food52.com/recipes/1572_sweet_potato_gratin_with_smoked_paprika_and_cayenne

A little oxidization won't hurt, I suppose. I also might leave them uncut but peeled.

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'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?