You can visit guest services and adjust the percentages of your gratuities goes to each area.
It is worth mentioning that many of the same workers who serve in the MDR also serve sea day brunch, lunch on Lido, etc. So while you may not eat in the MDR for supper - you're still being served by many of the same workers when you eat at Guy's, Lido buffet, etc.
It's rare that I meet repeat crew these days, mostly only when I sail on the same ship within months (they'd still be on the same contract). Regardless, just because I may or may not have removed grats on my last cruise doesn't mean they can immediately pocket or distribute the tip I hand them this cruise. They would still need to hold on to it to see what happens this cruise.