Is Boars bristle hair brush good to use on bleached and dry hair?

I have heard that brushing is bad for hair. Is that true? I also want to know from you guys whether it is okay to use boars bristle hair brush. I have even noticed that brushes that claim to be 100 percent boar bristle are different; some have firmer bristles than another boar bristle brush. So what would be better for hair, the brushes with firmer boar bristles or the softer one? Is Boars bristle hair brush good to use on bleached and dry hair? I would really appreciate your useful information. Thanks.