Grand juries are picked randomly from the jury pool. The prosecutor has no input into the selection process, nor does he have any for cause recusals available. Any prospect having connections to any of the principles in the case would be excused. The evidence, including the prosecutors arguments as to why the case should go forward, is examined, and the finding is made. IMO, that's about as impartial as the system can be made.
Since there are other ways for the prosecutor to bring a case to trial, the prosecutor is not apt to waste his or the GJ time in cases unlikely to be found winnable.