As the others have said, you should play at a lvel comfortable to you. It's important to play at the same level sitting and standing. There is simply no point in learning how to play two different ways.

You should play at a comfortable position with no strain on your wrists as Ren mentioned as well. I play with my guitar at a moderate to high level making sure that I don't have to stress my muscles to make any extra effort to reach the notes.