As Donna mentioned, it really depends on your activities and spending habits. Most of my spending is onboard (drinks/excursions/specialty restaurants) so thats all on a credit card.
For cash, I usually take $75 for each day of the cruise + $100 for my 2 travel days. That is used for tips on board (in addition to auto tips), taxis, and anyone else that would get a tip (porters, shuttle drivers, etc...)
I rarely buy anything ashore now and if I do, use a credit card and always end up having extra cash when I get home.