There's two schools of thought for correctly balancing your warbird's Center of Gravity when it comes to low wing models with retractable landing gear.
Do I balance with the gear up or gear down? Do I balance upside down or right side up?
Let's tackle gear up or down first.
If you spend some time researching the topic online you will find fair points for either side of the argument. The positive of balancing with gear up is that you're choosing to balance your model in its normal flying configuration.
The positive of balancing with the gear down is that you're balancing your warbird in it's most vital state (gear down for taking off and landing).
There's no perfect one size fits all answer here. Like many aspects of this hobby it comes down to preference. However, there are considerations to contemplate that may help you decide what is best for you and your model.
If you choose to balance your model with the gear up you need to be aware that when your gear is down this could shift the model's CG either forward or aft depending on the gear's location and you will need to counter that shift with either trim or a mix (Flight Mode) in your radio, or manually with the control sticks which isn't a favorable situation.
If you choose to balance your model with the gear down, while this will be a favorable location for takeoffs and landings, once you retract your gear the CG could shift forward or aft again depending on the location of your gear, and this occurs in the air with little margin for error. Like stated above you will need to counter that shift accordingly.
Whichever method you choose, it's vital to test your model on the ground to see how it reacts once the gear is either deployed or retracted based on how you chose to acquire your CG. From there if your radio supports it, consider a mix that will compensate for the CG shift.
Example: Let's say you choose to balance your warbird with gear down. When you retract the gear the model becomes tail-heavy. So in this case you would want to add some down elevator in your mix that will drop the nose slightly to compensate for the tail-heavy condition.
Now with low wing warbirds is it better to balance upside down or right side up? Again, there's numerous opinions on this and the truth is that it's a matter of preference. Regardless of what anyone says, there is no right or wrong method. It's all a matter of preference.
However, the majority will tell you to balance your low wing model upside down. The primary reason for this is because most simply find it easier to balance a low wing model upside down. As someone named "Engine Doctor" in a RC forum once put it;
"Remember that with low wing aircraft the actual CG is somewhere in the middle of the fuselage above the wing. Balancing anything right side up above your balance device (or fingers) will be like balancing a pea on a knife edge. Inverted, the actual CG hangs below the wing and is easier to control/balance."
So while you can still correctly achieve the proper CG right side up, it comes down to the idea that it is an easier process to achieve upside down but technically-speaking, yes you can achieve an accurate CG either way.
Hopefully you find this article helpful and it helps you achieve many rewarding and successful flights.