We generally don't tip the M'aitre D - when asked to do so.
If the "tip" is the service charge applied to your sail account, we usually leave it as is, tipping extra cash in person -where needed.
The one and only exception this was on our FOTS cruise last summer. To be honest, I can't recall exactly if he was the Head Waiter or the M'aitre D ... but this gentlemen went out of his way EVERY night to give exceptional service. EVERY night he greeted us, and then came by to check on us. If he saw our drinks were low, he made sure we were brought more. If he saw that we had ordered something that was not the "Chef's Special" he brought us some to sample, because he thought we might enjoy it (often he was right!). He also went over and above to get access to more than our allotment for the character breakfasts. We gave him a very generous tip, and he deserved EVERY penny. He also knew of my son's peanut allergies and gave us daily reports of any items that might be a concern.