Waiters and cabin stewards on Princess and NCL have told me that since gratuities have been added to the onboard accounts the "stiff" rate has dropped drastically. Many people who would just make a dine and dash on the last evening or not leave a tip for the cabin steward will not go to the front desk and have their gratuities removed.
Even with gratuities being added to the onboard account you can still give someone extra if you feel their service was excellent.
I actually like having the tips put on the onboard account and I still give extra to the people who go above and beyond in service. It's the best of both worlds.