Просто установите начальные значения из выбора поля, например:
MY_CHOICES = (
("some", "Some choice"),
("another", "Another choice"),
("best", "Best choice")
)
...
multiple_choice = forms.MultipleChoiceField(
label=u"Select multiple",
choices=MY_CHOICES,
widget=forms.widgets.CheckboxSelectMultiple,
initial=(c[0] for c in MY_CHOICES)
)