If I were you I would change the rear springs as well yes, and as Phil says, get the strut top bearings done at the same time. Doesn't Schnelltracker say something about changing the bump-stops also?
While you're in there, check/grease the ARB bushes.
The rear springs rarely break so unless you are planning to lower the car then its not really necessary. In addition, the rear springs are separate from the dampers and much easier to change if you need to replace them at a later date. Rear bump stops are prone to wear so be prepared to fit new ones when installing the FSD's (shortening the bump stops only applies if you fit lowered springs)