I'll try not to get into the tipping conversation. But the minimum amount is suggested for each position for the work that is expected of your serving team. Their work is providing you drinks at your table in advance of you sitting down, providing you with the food, clearing it away, and providing other Disney standard customer service.
When you request off items, you throw off their timing in terms of grabbing your meal from the 'order up' station. Disney makes a lot of its dinners at once, and when you have something that isn't already plated, that takes time. Or if you order something from another dining room, your staff needs to go to that location to pick it up. I don't know how the chefs and food prep folks get paid so not sure what 'good will' some of the servers need with the back of the house cast members. So, in my opinion you should tip what you think it is worth. I personally would think a couple of bucks for each request sounds reasonable for most people. I always plus up my standard tips anyway.