If you are on a ship that automatically adds the tips to your sign and sail card you can ask to have them removed or adjusted if you prefer to tip in cash. You would do this at the purser's office.
Generally tips run about $9.00/day/person. So that is $63.00/person for a 7 day cruise if you use the posted guidelines for tipping.
According to the carnival brouchure:
Room Steward: $3.50/guest/day
Assistant Waiter: $2.00/guest/day
tipping of maitre d' and headwaiter is at your discretion.