There is really no such thing as a sloped back. When the dog stands in a natural stance the back is level. When the dog is placed in a more extreme stow stance or "stack" the back will slope to the rear.
Picture a person just standing and then the same person set up to sprint. The perps's stance is different but not incorrect.
The GSD is a trotting dog and needs the propulsion from the rear to maintain the ground eating trot which is why the rear angulation is the way it is. It is a result of a longer thigh. Any over extreme is not good.
PTE,AC,URO3,AG2,UCD Xtra!Xtra! v. TeMar CDX,GN,RE,CGC,TC,HIC, Bh "Havoc" 6/4/07
PAM, URO3, UCD, UACH Tidmores Rising Star Lydia "Mayhem" CD,BN,RE,AX,AJP,OFP,P1J,CA,DN,HT,TKN,TC,CGCA 4/4/12
Lucky, Wolf, Max, Kayos - gone not forgotten.