Not positive about the federal grand jury, but it seems you cannot plead the fifth amendment. You refuse to testify they can put you in prison for contempt. Which is what happened to Miller.
You can be granted immunity from prosecution in exchange for your testimony.
You can say, I don't recall and they can do nothing to do for that. You must say something tho, not just clam up.
You most certainly can plead the fifth in a grand jury proceeding. Given the President's directive to cooperate with the special prosecutor, Libby's pleading the fifth would have led to his resignation/firing as Vice President Cheney's Chief of Staff. And it probably would have made an indictment on the substantive charges more likely, if Fitzgerald could have otherwise proven he knowingly passed on classified information to the press.