Excerpts from Alfredo Palhares's message of 2015-04-01 15:38:52 +0100:
Hey Alfredo,
There is no setting for this, however there is a crypto-mode hook that
is run when you start editing message. In order to answer your question
I just wrote this:
$ cat /path/to/hooks/crypto-mode.rb
crypto_selector.set_to(:sign) if crypto_selector.can_set_to?(:sign)
This automatically selects the :sign if it is possible.