you shouldn't take more than a couple steps back to avoid an onrushing opponent, no. You give too much momentum to a guy and if he catches you, you'll have problems. I noticed Jermaine Taylor did it early in his career but I never thought it would cost him like it did, the old adages have reasons for them. If you take a couple steps back, it's fine, but past that, be ready to do something else, maybe lunge foreward and punch, maybe move left or right but ya, that's bad. I don't think that's what we mean by fighting backwards though. I think it means just moving back to either make it harder for an opponent to get off or to make it easier for you to punch. Taller guys often need to back up to make punching room for themselves, Ali did it, Sugar Ray Robinson did it, Bert Sugar claimed that Ray knocked Fullmer out going backwards, well, it wasn't really going backwards, Ray took a step back, Fullmer lunged in but ray's feet were planted, it's often cited as the greatest kayo punch ever but i wouldn't exactly call that fighting backwards. His feet were perfectly planted. If a shorter guy is swarming in, there are several things you can do, you do like George Foreman did with Frazier, push him back, catch him coming back or like ali did with frazier in the first fight step back, get your punches off while frazier comes in, clinch when he gets inside/repeat.