> After President Gerald Ford left the White House in 1977, intimates said that the President privately justified his pardon of Richard Nixon by carrying in his wallet a portion of the text of the Burdick decision that suggested that a pardon carries an imputation of guilt and that acceptance carries a confession of guilt. Legal scholars have questioned whether that portion of Burdick is meaningful or merely dicta.
EDIT: However, as pointed out, the following statement, in my original response, was incorrect: Nor do pardons even have to be “accepted” to have effect, it's a unilateral executive power.
EDIT[2]: additional response on another point previously overlooked:
> however, is there any case where the admission of guilt has been used to coerce testimony from a pardon recipient?
Any imputation of guilt is a side issue on coercing testimony; a pardon (if accepted, and this is the main practical reason why it might not be accepted) makes the recipient immune to prosecution for the offenses it covers, and thus makes the Fifth Amendment protection against self-incrimination irrelevant to the extent related to offenses within the scope of the pardon. The government has plenty of power to compel testimony wherever the Fifth Amendment does not apply already, an admission of guilt is not necessary once that shield is moved out of the way.
https://en.wikipedia.org/wiki/Burdick_v._United_States
> After President Gerald Ford left the White House in 1977, intimates said that the President privately justified his pardon of Richard Nixon by carrying in his wallet a portion of the text of the Burdick decision that suggested that a pardon carries an imputation of guilt and that acceptance carries a confession of guilt. Legal scholars have questioned whether that portion of Burdick is meaningful or merely dicta.