Well, when Starbuck died and was reborn, the people who took her brought her to Earth as the first order of business. I would rate that this means:

• Most probably, they are from Earth, and brought her to their HQ
• Alternately, they are not from Earth, but are chums with Earth, and brought her there to meet the folks there.
• Possibly, Earth is now abandoned and a good place to settle, they took her there because of that
• Highly unlikely, Earth is vulnerable to a Cylon attack, and they showed her how to get there where the Cylons could follow