As above, this was started in 2011 on the Magic due to issues of guests not tipping on the Med. It was expanded to all ships about a month ago. Guests are notified of the procedure with a letter in the stateroom (we got ours on the first night).
You do have to ability to go to GS and remove the automatic tipping as well as to increase or decrease the amount. You can also increase it by leaving the automatic in place but add some cash to the envelopes. The servers do not know whether you have altered the automatic until the last night, so it has no influence on the service you will receive.
I believe that I have only tipped less than the "suggested" amount once on a
DCL cruise as the service we receive ranges from above average to fantastic. Thus, I let the automatic stand and adjust it on the second to last day-just take the tickets you get in your stateroom to GS. You will complete a form indicating the amounts you wish to apply to your account.
I don't find that it makes any difference. We've done several Magic cruises with the automatic and now one on the Fantasy. If it matters to you, you are still fully in control of the tips.