I think it would depend on the crime. Things like Winona Ryder's shoplifting don't matter much to me; she was, rightly, taken to court and sentenced, but once the sentence is served, there's no need to hold it against her IMO.
Also, even with a violent offence, which I'd have a problem with, it would depend if it was a one-off or not. Cheryl Cole has never repeated that ugly little incident with the toilet attendant, whereas Naomi Campbell has a long record of aggressive outbursts. Therefore I'd boycott Naomi but not Cheryl.