I think different people will have different takes on this question. I know a Frenchman on this site who believes that italki is a community, and he prefers to address everyone using "tu." Therefore, in this spirit, it would probably be equally acceptable (to him) to use "Salut" with everyone.
However, perhaps it is safer to use a less informal greeting such as "Bonjour" until the native French speaker addresses you using casual speech first. In real life, this is how things would be done anyway … start with a level of formality until the other person chooses to use a more familiar discourse pattern.
