Every passenger, whatever dining option they choose is charged $6.50/day. Even my 10 year-old granddaughter's account had that charge and I had such a difficult time getting that transferred to my account.
You can go to the purser's desk (they say any time during the cruise, but actually it has to be the last day because you can only change those charges already on the account) and change those charges. You can have them removed, add to the amount, or decrease the amount.
ALSO, you can give cash tips, without an envelope, or with an envelope. Cash-free cruising??? Just hold out a couple bucks anywhere and see how cash-free it is!
Also, you can go to a specific bartender and buy a tip chit, charged to your shipboard account (in addition to the $6.50/day), and give that chit to your server.
None of this goes to your room steward. You tip him directly with $$$ or a chit which you would buy from that same bartender, charged to your shipboard account. You should also tip the room service guys on a time to time basis. If your room steward brings your room service orders, remember he does so as an extra in his busy day...so use your conscience with that.
An aside...I have always preferred to tip cash where possible. Then the server can handle that money however he/she chooses, with no record if she/he wishes. But that is only my personal preference.