We did tip some positions extra - here's what we did on our last cruise:
Dining Manager - We weren't celebrating anything special, so we only saw the guy twice. He stopped by for a long chat on tip night (surprise, surprise) but I went ahead and tipped him the $13.00 or whatever the recommended amount was. I wasn't too thrilled about it, but figured it was only $13.00, so I wasn't going to make a stink about it.
Stateroom Host - He was OK. He was efficient and friendly but didn't do anything "above and beyond". He was supposed to be tipped $56.00 or something like that. I rounded it up to $60.00
Assistant Server - He worked very hard, and was very efficient. He was nice enough, but not very friendly. He didn't really try to make conversation or anything, but he did a very good job and we didn't want for anything. I think he was supposed to get $38.75, which seemed ridicuously low for the amount of work he did. We left him $75.00
Lead Server - Our lead server was fantastic. He was friendly, accomodating, efficient, and very thoughtful. He complimented my dress on formal night, he brought us surprise appetizers and desserts to try, he chased my husband into the hall one afternoon to check up on me as I had been feeling a little queasy the night before, he provided an informed and helpful run down on the menu each night, he enquired about our activities and plans and made helpful suggestions, and all sorts of other nice things. He was really a gem. Again, the $56.00 he was supposed to be tipped felt ridiculous. I couldn't live with the idea of leaving him only $56.00 for everything he had done all week. DH and I figured that if we had gone out to a similar type of formal dinner at home, the check would have been about $100. For service like we got on the ship, we would have tipped at least $20.00. So, we multiplied $20.00 times 7 nights, which came out to $140.00. We rounded it up to $150.00, and that's what we left him.
That's what worked our and felt right for us. Your mileage may vary...