On the forehand side I like to see the elbow tucked in because it provides a little bit more stability. Having said that, keep in mind that probably two-thirds are going to be on your backhand - I like the elbow up on that side for several reasons. However, some teach differently on this - I get very good results so I'd be reluctant to change at this point.
Now, as with most other shots, this isn't a hard and fast rule that can be applied to all. I just find, especially on the FH side that when the elbow wanders away from the body, the player starts taking the ball too late, has a tendency to punch the ball long, mishits often, cannot change direction, lets the racquet head dip and so forth.