Until you're comfortable playing in a lot of different positions, Schlegel's suggestion is a practical one. It'll give you a consistent reference point. That way, you won't learn how to play sitting down, and then feel out-of-place when you finally play live. (And playing live, you want to be as comfortable as possible!)

You might notice that some very well-known guitarists play with their guitars all over the place (hanging by their knees, up high, etc).

You'll eventually find that some positions make it easier (or harder) to play different types of things. For instance, if your guitar is too low, it'll make you bend your wrist / wrap your thumb around the neck more. Then again, it can be a lot of fun to play that way, if the song is easy.