This answer is written from my perspective as an American.
I think that usually, saying "you can" is likely to sound rude. If you say "You can get me the check," it may sound like you're sarcastically saying, "I want the check, but since you haven't given me the check, you must have been unaware of the fact that you can get me the check."
On the other hand, it may sound like you're giving the server permission to give you the check. This is also bad, because the server doesn't need your permission to give you the check, and it's impolite to imply that they do need your permission.
I can't think of any time that I would recommend saying "you can" to a server. Even when I'm telling a server that they can keep the change as a tip, I say, "Keep the change, please."
A good rule of thumb is that if you're making any kind of request in the United States, you should always phrase it as a question.