ssl_cipher_list

[dovecot at ptld.com]

Is there a difference between not setting
    #ssl_cipher_list =
or setting to nothing
    ssl_cipher_list =
or setting to
    ssl_cipher_list = PROFILE=SYSTEM

Which is the best preferred way for default?

Also, the docs say "You should usually prefer server ciphers and their 
order, so setting ssl_prefer_server_ciphers=yes is recommended." however 
this setting is =no by default. Why? Should it be yes or not?