If that is the case then you can modify the following file:
include/staff/viewticket.inc.php
After value="mine" remove 'checked' from this code:
<input type="radio" name="signature" value="mine" checked <?=$info=='mine'?'checked':''?> > My signature
And after value="dept" add 'checked' here (no quotes):
<input type="radio" name="signature" value="dept" <?=$info=='dept'?'checked':''?> > Dept Signature