The default for member data should be false since this is the default for all the other calls. It'd be weird to have one call where it's reversed.
It would be good to follow the convention for the other methods and have top_member(...), top_member_in(...), bottom_member(...) and bottom_member_in(...).
Could you also add some information to the README on the usage of top_member(...) and bottom_member(...)?
This got me thinking. What do we think about making Leaderboard Enumerable-friendly?