I do not erase previous owner's names, address, etc.
The only thing that I erase are prices from previous sellers (think garage sale or thrift store) Usually those are written in pencil lightly and erase easily. If the paper is colored and will show an eraser mark of light/white where the color has been removed, I leave the pencil marks.
I guess you could say my policy is, each book is a judgement call as to whether I will do more harm than good.