The good morning may be the best assistance exercise when it comes to improving the squat and deadlift. It hits the posterior chain hard and forces you back to maintain a huge amount of isometric force throughout the back.
To me, the more range of motion you can reach while maintaining solid form, the more beneficial the exercise will be. I don’t think it’s terribly important to go super heavy with good morning variations. Focus on quality over quantity and perform the exercise with perfect technique on each and every rep.
Here is a demonstration of good mornings performed with a straight bar. Notice the flat back, the slight knee bend, and the butt moving back horizontally.
Good mornings can also be done with a cambered bar. It’s hard to explain, but using this specialty bar feels quite a bit different. Check it out here:
If you want to get better in the big 3, you could probably benefit from good mornings. They’re a great assistance lift when used properly.
However, don’t try to go too heavy with this lift. Keep the load relatively low and stay around a medium rep range (6-12). Of course it’s acceptable to go heavier occasionally, but personally I wouldn’t go below 4 reps with any good morning variation. The risk just isn’t worth the reward at that point.
Whether you use them or not, the good morning is a great supplement to any powerlifting friendly program. Just be easy and stick to moderate loads for medium reps.