I'm playing a Star Wars Saga Edition game right now where my character is a former privateer fighting in the Jedi Civil War for the Sith. He was fairly honest with his party members about his former criminal affiliations, that's how he met them in the first place. The fact he was on the wrong side of the war only came out when a conversation about the war came up and he was directly asked about it. The Jedi in the party took it surprisingly well, but that's probably more due to the conversation being completely unserious other than his admission.

Lesson learned: you can probably trust your party with your dark past.