in the late 80's and early 90's, when there were still serve and volleyers on the women's tour, graf had to hit a lot of topspin backhands. whenever she was playing well, as she did all year in 1988 and also in 1989, her topspin backhand worked wonders. it has often been said that her topspin backhand was even better in practice.
however, during the instances that she wasn't at her best, her opponents still had the most success attacking her backhand. this was evident in 1990 at wimbledon and the us open, and also at the 1991 australian. in those events, steffi lost to garrison, sabatini and novotna respectively, because they attacked the graf backhand. even so, graf had her chances in those matches.
throughout the 90's, graf's main opponents were baseliners - the likes of seles, sanchez-vicario, martinez, fernandez, capriati, pierce, davenport, huber, coetzer, majoli, and, at the tailend of her career, hingis, venus and serena. her slice was the shot that worked best against them in rallies because she could work the point to set up her forehand. so therefore, she really didn't need to hit a lot of topspin backhands. novotna and tauziat were the only serve and volleyers left and, yes, they did force steffi to hit topspin backhand passes. but graf had such a huge mental edge over them that it didn't matter what they forced her to do.
the other thing people don't appreciate is that graf also improved her slice backhand a lot since her earlier days. her slice wasn't always that good. when she lost to sanchez vicario and to seles at the french open in 89 and 90, her slice was relatively harmless, especially on clay. but she stuck with it and it became such a solid shot that she just wouldn't miss with it. it stayed so low and was so penetrating that it set up her forehand perfectly.