Regardless of whether or not you feel like tipping above the amount of included gratuities....the MOST important thing you can do if you have exemplary service from a crew member, is mention them in the post cruise survey. I have been told by several of them that this plays into their bonus pay and time off.
I'm 'in the middle' on the additional gratuities. If I have a steward, server and/or bartender that provides better than average service, I might give them a few dollars at the end of the cruise. Some cruises I find no one that fits the bill. Last fall on the Enchanted, I nearly ran out of my tip cash as there were so many that were outstanding!