9-11dKH is supposed to be the "normal" range, but as James suggested, if everything is doing good leave it alone. Stability is key, fluctuations cause color loss and tissue recession.
There are many reasons why people have different values such as the case of carbon dosing. Some people report burnt tips with higher alk levels, yet others don't.
There are also some claims that lower alk levels allow a greater uptake of calcium, though I don't know how true this is.