I agree with you about the servers not giving the soda card people "priority". It does great annoying at times. The price of the card is supposed to include tip, but how can they do that if they don't know how many sodas you'll actually get? So your extra tip idea is fine but that only works if you use the same bar or server each time.
We don't normally get the card as we just don't drink that much soda, but when we did, I would normally order a drink when my kids got a soda anyway which helped insure speedy service.
Oh, and as Dave will tell you, the extra tip to your "chosen" bar's crew always helps with the service.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013