As this video
illustrates, creating a repo online first is the usual way to go.
Release Notes do mention for SourceTree 1.5+:
Support creating new repositories under team / organisation accounts
So while there is no "publishing" feature, you could create your online
repo from SourceTree.
The blog post "SourceTree
for Windows 1.2 is here" (Sept 2013) also mention:
Now you can configure your Bitbucket, Stash and GitHub accounts in
SourceTree and instantly see all your repositories on those services.
Easily clone them, open the project on the web, and even create new
repositories on the remote service without ever leaving
You’ll find it in the menu under View > Show Hosted Repositories, or
using the new button at the bottom right of the bookmarks panel.