side screw usually very little or no preload. Especially if you use hard spring like silver.... center spring adjust it so the chassis is flat with rear pod. If center a little higher than rear pod will give a little more rear grip. But the car is less "predictable" when the rear end start to loose grip.
Side sprin is very funny, You can chose useing preload or not, And with preload y ...
ya... just personally I don't really like to add lot of preload as this makes the steering too fast, not very stable. I prefer to change to harder spring if necessary. Just my 2 cents