I cut the rear C springs and I regret losing the flat coils. But I had no choice because I had too much rake, which was only compounded by larger diameter tires out back, and some weight reduction.
If you decide to cut, consider where the spring will terminate in relation to the pigtail. With my pigtails properly positioned on the control arm, the top end of the spring sits in the unsupported portion of the spring perch, which is no different than if I had cut off more length.
Removing the flat coils means my isolators are getting gouged something fierce. Grinding a flat spot on the end helps, but only so much.