Not necessarily a hard fast rule, though suits generally look better without belts. The formality and clean lines of a suit are broken up by a belt and therefore bring the overall look down in many peoples' opinions. I tend to agree, however I have seen belts (particularly very slim belts in some sort of exotic skin) go fine with suits.
All and all I prefer braces over belts. The do a better job of keeping your pants up than a belt and at the same time do not disturb the harmony of a look.