The past pardons by past presidents were always long after the facts. The perps such as Michael Murphy had been convicted, served their time, and gone on with their lives. Mostly symbolic clemency and an expungment of record for rehabilitated offenders.
AFAIK, virtually none of them were part of an ongoing criminal investigation involving national security and the pardons weren't part of a coverup and subversion of the US judicial process.
Even Marc Rich, who didn't deserve a pardon IMO, fled the country and was an exile for years in Switzerland.
You can bet Lewis will get a pardon in January 2008. Another poster pointed out that if he was pardoned now, he might be compelled by immunity to actually tell the truth to the prosecutors. Can't claim the 5th after a pardon.