In "The Case of the Cross Dressing Carp" when they found the body in the remnants of The Rampart, Sam Braun's casino, Catherine handed the case off to the Swing shift Voluntarily.
Of course, this was after she got nailed a couple of times for staying on Sam Braun cases when she was either explicitly asked to stay away, or when she knew that she shouldn't have worked the case.
Otherwise, I can't think of any other cases where a CSI voluntarily recused themself from a case, even if there was conflict of interest.
On New York, I know Hawkes got tossed from a couple of cases, one because he knew the victim. The entire team got pulled from the case where Hawkes was accused of murdering the bartender.