I play as a winger, and drift all over th pitch to create and/or take chances. The middle of the pitch is the best place to park if you want to get longer shots on net, or to chip through to your stikers, but if you want to score a lot of goals you are best staying up front, taking up central or just off center positions. Your teammates will find you if you are in a good position.
Personally - I think that being out of position is no problem for ratings. Even if it is, it's better for your ratings to be involved with play, score goals, etc, than it is to just play your role perfectly, but like a mouse. One of the most effective movements in BAL, I find, is to switch wings to the wing where the ball is and make runs for through balls that are towards the wing you're not supposed to be on - in other words, it can be extremely effective to play out of position, and I haven't noticed that it hurts my ratings.