Nope, it can b ignored or deemed to be not relevant in the case, it does not have to be taken as down and dirty fact.  If that was the case every defence lawyer in the land would cite a precedent and get their client off scott free.  It doesn;t happen like that as each case is very individual and no two will be similar enough for any precedent to ever be wholly effective.  Also in trials that invole juries then you can site what the heck you want if it goes to the full length and the jury are sent to reach a verdict it is up to them what the result is and not up to the judge or the lawyers and, believe me, that often hinges on the courtroom performance of the witnesses and the defendant more than it does any previous precedent.