It's wrong to think open or closed stance. It can hamper your footowrk.
If you think about getting your foot lined up behind the ball, you are closer to the proper weight transfer.
Now which foot is that? Optimally it is closed. Easier to get your weight going into the court, but with more modern tennis you can obviously transfer weight more laterally and be fine.
So when to do what? That is really the question. The answer is, whatever you are allowed to do during that shot. If a guy hits a weak, short ball to my forehand, I am most likely going to drive with it all my weight moving forward so I can have a nice controlled swing and hit a winner.
If he hits wide to my outside there is no way I can be balanced and hit closed unless my name is Djoko and I possess elite ability.
The more I thought about balance and weight transfer and less about open vs closed stance, the better my footwork got.
Perfect example of why it is better to think this way. Get pulled wide on the FH side, hit a FH with an open stance..start crossing back to middle, opponent hits ball short but where I was so im wrong footed. Open stance can't happen and be on balance (short ball) so I hit the shot stepping in and closed because its all I can do. Doing this keeps me balanced, and I hit an easy CC winner. tough to describe each scenario, but the less I think aboout open or closed, the better my balance is.
Also, Agassi hit a ton of 2 handers open when pulled wide. Just watch his french open match against courier. You will see it in the first few games.