Where do Net Players Stand in Return Games
While level has some correlation to where you might position (especially when the serves are coming really fast) , I think it's better to understand what the purpose of each position and it's advantages & disadvantages regardless of level of play.
First it's important to understand that in doubles, the #1 most vulnerable spot is always between the net player and the baseline player. This is why you try to avoid the opposing net player because they have the best position to hit diagonally right at this spot. More on this later.
There are basically 3 positions for the returning team.
Net player around middle of service court level ( the typical doubles position )
This is a really aggressive position and also very risky for the returning players. The main problem is that your returner HAS TO to hit a good return cross court and keep away from net player. Because of how close you are to the net, it really emphasizes the weakness of that diagonal hole between you and your partner.
What's worse is that if your partner wants to hit an aggressive down the line or blast the opposing net player, all the net player has to do is poke it between you guys to neutralize the ball. Your partner, the returner, has to chase the ball down because it's behind you and you are close to the net.
If you were a few feet back you can return the ball back to the server without compromising the returner's position. This is the #1 reason why this return position starts disappearing the higher level you go. You want to give your partner the choice to go either direction and be able to cover the ball if the net player hits between you and your partner. This is impossible if you are standing too close to the net.
The main benefit of this position is , if the return is strong & cross court, you have the immediate option of poaching and are close enough to the net to hit any balls if it comes down the line. When you are slightly behind the service line during the return, it will take probably 1 ball rally length to get close enough to be in poaching position. As you can see the advantages aren't that good which is why this should almost never be your return position. Early on it's ok, because balls are slow and "loby" so it's not that punishing. There are mistakes everywhere and this one mistake isn't as glaringly obvious as others.
2) Net player slightly behind service line.
IMO this should be the standard returning position for most levels. As explained above, by standing in this position, you allow your returner the option to go down the line while covering the vulnerable middle position between you and your partner.
The main weakness of the position, is that once the return goes cross court back to the server, you usually will be still around the service line. So it's imperative that you cover the ground pretty fast and get as close to the net as you can. Usually this rules out poaching on the first ball that the server hits after he hits his serve.
Also for many players, hitting volleys around the service line is an uncommon and tough shot so this position can leave you vulnerable. I have seen many times, where players go on autopilot, and get stuck near the service line after the return. The server sees this and hits a ball down the line and the net player usually messes up. Something to keep in mind.
3) Both players back
This position gets more common when serves start getting scary, and any vulnerable return gets destroyed by the net player. Remember, two back versus two net players is an extremely bad position for the baseline players. Adept net players win this every time. The reason high level players resort to this tactic, is that the net player on the returning side tend to get pelted after 100 mph+ serves. This position gives the option to get 1 more ball back and try to get into the point. Very often you'll see the net players run to the net, if the return is good enough.
There are times, when normal tennis players, can use this tactic to their advance. I have seen it work in the high school level, just simply because net players just don't have the ability to fully take advantage of their position, and the returning players are good baseline players ( or inept at net).
Again there is no always or never. Always consider your position and options. Let me know if you need me to clarify anything.
