I used to be very much into saltwater reef aquariums, and using non-food-grade buckets or trashcans was always a huge NO-NO..but the real reason was the salinity of the water would break down the plastic, nothing noticible to the human eye, but it would leach toxic compounds into the water. Even then the compounds that were leached were rarely enough to harm fish, but the expensive inverts in the tank would be poisoned.
That being said, i don't know how much the acidity or basic qualities of wort would cause a leaching, and even at that I don't know if it would be enough for the human pallet to discern an off-taste.
That being said if you you can find food grade buckets you will sleep easier. The bakeries around here typically have buckets and buckets of icing that they will give away or sell very cheap.