Some experts (eg Brazelton, he has a great book on potty learning) recommend waiting until ALL signs of readiness are present, eg being able to take her clothes off and on, waking up from naps dry, etc. Personally I don't think there's harm in letting her sit on the potty if/when she wants to, but don't assume that means she will progress at any particular rate, or be done earlier than a kid who starts later. My oldest daughter was interested at 18 mos, but not out of diapers in the day til after her 3rd birthday.
Also, keep in mind that while there may be general trends ("girls are easier than boys"), they may not apply to YOUR kids. They're all individuals, and the only guarantee is that nothing works for every kid. :)