Balancing springs were used a lot on retracts with large wheels when servos weren't quite as powerful. Basically, you want the spring to be strong enough to balance the weight of the main gear when it's about 1/2-way retracted or slightly more. When extending the gear, you'll be 'fighting' the spring a bit, but not the entire load because the weight of the gear helps you. You can tell you need the assist spring because the servo strains when raising the gear. Lowering, though, should prove to be no problem.

Dave Platt, in his old RCM book, "Scale in Hand", discussed the topic. He may also say something about it in his current series of videos, but modern servos and pneumatic systems are strong enough to raise and lower all but the largest of retracts.