Everywhere else in MW, we agree on ~~~~ being used to generate a signature, but somehow on the editor toolbox the button has always went a slightly different way, outputting --~~~~ instead. I, for one, either type the signature code directly, or if using the button, always use user JS to override its output.
I think we should either remove that extra -- or at least make it an option on Special:Preferences which could be selected by default, but could be turned off by users like me.