via Think Progress:
There's No Precedent Barring White House Aides From Testifying
According to the Congressional Research Service, under President Clinton, 31 of his top aides testified on 47 different occasions. The aides who testified included some of Clinton’s closest advisors:
Harold Ickes, Assistant to the President and Deputy Chief of Staff - 7/28/94
George Stephanopoulos, Senior Adviser to the President for Policy and Strategy - 8/4/94
John Podesta, Assistant to the President and Staff Secretary - 8/5/94
Bruce R. Lindsey, Assistant to the President and Deputy Counsel to the President - 1/16/96
Samuel Berger, Assistant to the President for National Security Affairs - 9/11/97
Beth Nolan, Counsel to the President - 5/4/00
In contrast, between 2000 and 2004, Bush allowed only one of his closest advisers, then-Assistant to the President for Homeland Security Tom Ridge, to appear in front of Congress. He has also refused three invitations from Congress for his aides to testify, a first since President Richard Nixon in 1972. Clinton did not refuse any.
http://thinkprogress.org/2007/03/20/white-house-testify/