Initially Henin was imbalanced. Her backhand was always solid but it wasn't until she got older that she improved her forehand and became a more balanced hitter.
Davenport was also fairly balanced, although her forehand was perceived as her money shot she could do damage off of both wings. She did not earn all her ball striking praise solely on her forehand. If she was fitter and moved better she could have done a lot more.
Mauresmo was also somewhat balanced. Her backhand gets a lot of credit but when she was on her forehand was stellar...sadly it was prone to breaking down in clutch situations.
Dementieva was also balanced in this area...her serve was her kryptonite but her groundies and returns off both wings were very impressive.