They should riot if the Grand Jury acquits. It tells them what they have suspected all along, that their police are above the laws, and that protesting is not going to change that.
The Grand Jury is not a criminal trial, it is just a formal hearing to see if there is reason to bring charges against the accused. There are 6 eye witness accounts that claim he killed that kid in cold blood, even if a criminal trial finds that they are mistaken, or that their was unusual circumstances that lead to an acquittal, all a Grand Jury needs is probably cause, the same level of suspicion that the cop needed to stop Brown in the first place. There is undoubtedly enough for probably cause. The very fact that the Grand Jury has taken so long to see all the evidence tells us so.
This extended Grand Jury session has been a ploy to allow tensions to die down so they can sweep it under the rug, except tensions haven't died down and now they just going to sweep it under the rug anyway.
Based on what you think you know, they shouldn't acquit. But if the grand jury hears all the facts and decides to acquit, won't you then reason to accept that maybe you don't have all the evidence?