Rest your weight but stay tight...
I'm with Fazc...I don't like box squats (or GM's) because they groove a forward leaning squat pattern. If that's what you want, good, but if you want an upright squat, squat upright...I always prefer low-bar, wide-ish stance and a depth that doesn't round the low-back.
I think the box squats and GMs are usally intended to make you stronger when a heavy squat folds you over, but the training thresh hold is not worth the risk IMO.
If you box squat, front squat, GM, etc... keep it lighter as assistance and worry about adding pounds to your 100% kosher squat.
"You must have a very strong reason for doing an exercise. If you don't, scrap it and move on." -Jim Wendler