As a server, I see that as a little extra to say I did a great job. Some poeple give extra and some don't. For some people that would only tip 15% usually, they feel that this is already above and beyond and for people that tip 20% on a regular basis, they will add a little extra. When I was using the DDP with my husband, I found very few ocasions where the service justified an extra tip, and when it did, we left an extra $5-$10.
I think it is great that you are aware of the clean up required after a "messy" table! I can't tell you how many times people have left the table (and floor, and sugar caddy, and chairs) in complete disarray and then leave under 15%!! When dining with my nephews, I always left a little extra knowing that the server was goign to have to spend a little extra time cleaning up!
Jocelyn