Thanks zentara. Actually I found optionAdd under "perldoc Tk:option" (not plural). Usage wasn't exactly intuitive or clear, but I came up with the following which does what I want - sets default values for buttons without affecting any other type of widget.

$mw->optionAdd('*Button.font', '{Arial} 14 {bold}', 'userDefault');
$mw->optionAdd('*Button.padX', '2', 'userDefault');
$mw->optionAdd('*Button.padY', '2', 'userDefault');
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