The current version of the plugin does not have a mechanism for that, but you can modify that simply enough.
In version 2.0, line 85 is:
echo ' <p style="line-height: 120%; margin-top: .5em; margin-bottom: 0;"><input type="checkbox" name="auto-tweet-checkbox" id="auto-tweet-checkbox" onclick="if (this.checked) { document.getElementById(\'counter\').style.display=\'inline\'; } else { document.getElementById(\'counter\').style.display=\'none\'; }" />' . "\n";
Change that to:
echo ' <p style="line-height: 120%; margin-top: .5em; margin-bottom: 0;"><input type="checkbox" name="auto-tweet-checkbox" id="auto-tweet-checkbox" onclick="if (this.checked) { document.getElementById(\'counter\').style.display=\'inline\'; } else { document.getElementById(\'counter\').style.display=\'none\'; }" checked="checked" />' . "\n";
Note that the only change is the addition of checked="checked"
to the end of the input tag. (Mind the extra space as well.)