This is what I have heard also. Most of them send the money back home so it is easier for it to just go on their accounts.Most of them actually prefer the tips going through the onboard account. They don't have need for large amounts of cash while onboard. So while they would happily accept a gratuity either way, it saves them having to deal with cash.

I never understood why if you add it to your account you then have to give the slips to the servers. Why is this not done automatically for them?
They do automatically get the monies deposited directly to their account. The slips help them reconcile what they should expect to receive. Kind of like comparing your paystub to the amount deposited to your bank account. If a passenger forgets to give the little coupon, the servers will still receive the tips that were charged; they just won't have that record of how much to expect.
Enjoy your cruise!
What I do not like is the last two times we sailed we returned to our room to find the slips and envelopes and account printout on our bed after turn down service -- which means our room steward saw any additional amounts we may have added (that's why we only do cash now for additional tips). I just feel like that type of item should be put in an envelope for privacy sake.
You can always add the additional amount after you got the slips. On all my longer cruises, the sheet came at least the evening before the last full day (on the Panama Canal cruise I think it was even two days before debarkation). So there is plenty of time for a quick stop at Guest Services to have the amounts adjusted. We then got new tickets printed out there with the new amounts. This way your stateroom attendant does not see anything.

