I managed to avoid registration with the current version as of today, which is 3.3.8.
All you need is yet an already mentioned 'accounts.json' placed in
%AppData%\Atlassian\SourceTree\accounts.json (look carefully, it's now %Appdata% not %Local...%)
The file I used prepares a GitHub account with OAuth (you will need to apply some fresh OAuth from the edit accounts dialog). You can also delete that account and do what you like.
[
{
"$id": "41",
"$type": "SourceTree.Model.ScmAccount, SourceTree.Api.Host.Scm",
"IsDefault": false,
"Authenticate": true,
"HostInstance": {
"$id": "42",
"$type": "SourceTree.Host.GitHub.GitHubInstance, SourceTree.Host.GitHub",
"Host": {
"$id": "43",
"$type": "SourceTree.Host.GitHub.GitHubHost, SourceTree.Host.GitHub",
"Id": "github"
},
"BaseUrl": "https://github.com/",
"Protocol": "HTTPS"
},
"Credentials": {
"$id": "44",
"$type": "SourceTree.Api.Account.OAuth.TwoZero.OAuthTwoZeroCredentials, SourceTree.Api.Account.OAuth.TwoZero",
"AuthenticationScheme": {
"$type": "SourceTree.Api.Account.OAuth.TwoZero.OAuthTwoZeroBearerAuthenticationScheme, SourceTree.Api.Account.OAuth.TwoZero",
"Name": "OAuth",
"Description": "OAuth Token",
"HeaderValuePrefix": "Bearer",
"UsernameIsRequired": false
},
"Id": null,
"Username": "github-username",
"DisplayName": "Your Name",
"Email": null,
"AvatarURL": "https://cdn3.iconfinder.com/data/icons/inficons/128/stackoverflow.png",
"EmailHash": null
}
},
{
"$id": "18",
"$type": "SourceTree.Api.Host.Identity.Model.IdentityAccount, SourceTree.Api.Host.Identity",
"IsDefault": false,
"Authenticate": true,
"HostInstance": {
"$ref": "42"
},
"Credentials": {
"$ref": "44"
}
}
]