DCL is one of the few cruise lines that actually still allow you to control the tips. Most of them add a standard set of tips into the price of the cruise up front (which in some instances does not give the staff much incentive to perform at their highest standards). DCL also simplifies the process by allowing you to pay tips at the pursers desk. They actually encourage this, because you can simply charge the entire transaction to your cabin, and they can make direct deposits into each CM's account.
Now, how you go about deciding what to tip is a personal decision and you may want to go up or down from the standard based on your experience during that sailing, but
The generally agreed upon standard works like this:
Room Steward - $4.00 per-person/per-day
Head Server - $4.00 per-person/per-day
Assistant Server - $3.00 per-person/per-day
Dinning room Manager - $1.00 per-person/per-day (optional)
So
for two people sailing seven days it would be $24.00 per day or a total of $168.00.
Heres a site that will calculate the average tip for you:
http://cruisetip.tpkeller.com/
If you order room service, you would generally tip a couple of bucks to the CM that delivers the order. We take an envelope with a bunch of dollar bills in it and keep it somewhere near the cabin door.