Yes, you have the right to refuse to cooperate with the grand jury and/or the federal prosecutor. This decision should only be made after consultation with your attorney. In some cases, cooperation may be beneficial to you as a means to avoid indictment, to secure a favorable plea agreement, to avoid jail time or receive immunity. In other cases, cooperation would be a detriment to your defense.
If you elect to speak before a federal grand jury, you would be required to explicitly waive your 5th Amendment privilege against self-incrimination and consent to full examination under oath. These responses would be on the record and used in any future prosecutions against you.