The issue was just that I had hardcoded defaults for the SSL certificate names based on nginx_server_name - which hasn't been set yet, thus running the proc when the "if" statement is evaluated in 'set_default'.  I've gone back to just asking user for paths if using certs. 

Fixed in https://github.com/alexagranov/capistrano-nginx-unicorn/commit/afc35cfefbd9b3ee890d2632210bea4af4e078e3