The reason servers get pissy is because it can be a huge pain in the ass to separate checks after the fact, especially if it is a large table. Servers don't always remember everything that every single person ate or drank. If they just have a big list of items, trying to figure out who had what can be really annoying.
If you tell them at the beginning, they can ring in everything separately from the get go. Makes it a lot easier at the end of the meal.
When I was a server, I would always ask if my customers wanted separate checks before I ever started taking an order.