Backspacing for 9inch rims

im looking at getting some 18"x9" rims now my question is what backspacing should i bee looking for??
ill be runnning a 275/70/18 duratrac tires
I know everyone says 4.5 backspacing but isnt that to much on a 9" rim??
i ran 5" backspacing before and i rubed slightly only at full lock on my frame above my lowers control arms

 

Assuming you have a 3" lift you should be able to get away with between 4.5" and 5". I'd still expect rubbing though, as thats over a 33" tire.