I scored about 12 frosting buckets over a couple years and currently have grain stored in at least 8 or nine of them. I cleaned them in oxyclean and hot water then starsan. All of them seemed to either retain a very slight hint on the inside or maybe since I didn't clean the outside as well that some of the smell tainted my schnozz from the outside of the bucket. Either way, I have had open Maris otter and 2-row as well as specialty grains and none of my beer ever tasted like frosting. Lol. Hope that helps.
Keep in mind I'm talking a very slight hint, if it wreaked, I wouldn't do it.
Another thing is that the only thing that stored really long in my buckets are 1 or 2 pound increments of some specialty malt like black Prinz or chocolate and they are in seperate ziplocks.