Who have you booked with Ian? A lot of the Uk operators offer this and yes it means you have paid the recommended amount - you can decide to add more if you want. What happens with us (done 4 Magic cruises so far) is that usually on the Thurs our stateroom attendant leaves envelopes in our room with the vouchers showing the tip amount - you just put the vouchers into the appropriate envelopes.
Wendy