If you choose MTD you must pay for your gratuities in advance. Most who choose MTD do so before the cruise commences in order to secure the best times.
If you booked your vacation in CANADIAN dollars and choose My Time Dining (MTD) before the cruise be aware that RCCL converts the US gratuities at a rate of 1.25. The rate RCCL will charge you for MTD if you wait until you board will be a more appropriate 1.04. That is an extra $25 for a one week trip for two if you pay before the cruise instead of after you board.
The following comes from the RCCL web site: "Exchange rates are determined by a competitive pricing program".
I have contacted RCCL one month ago and although they admit that the rate is far from competitive and they feel that it is probably an error they have yet to do anything about it.