I guess it depends on where you live and what tax you pay. I pay 14% tax so pre tax or after tax can make more than a couple of bucks difference. I always figure out the tip on the pretax amount and I am glad to see disney do the same.
I have a friend going in August with her family of six (eating mostly buffets) and I have helped her figure out her tip amounts based on the price of the meals (before tax).