You can and should flatten the back or at least enough to remove the burr. The use of Ruler method that is up to you. I don't use any ruler method for the back of irons are flat. In the old days when the back of plneirons weren't flat, than I would understand the use of the ruler method. That said whatever gets you to work quicker.
Mark, It seems to me that the ruler trick is handy when restoring a vintage blade that you don't want to spend an hour flattening, but not useful otherwise. A L-N blade should be already flat, so the ruler trick gives no benefit, and once done requires you to use it from then on. I don't have a chisel plane but have plenty of other L-N planes, and I've never needed to spend any meaningful time flattening the back on any.
I guess my answer is redundant with Freddy's, but two opinions are better than just one, hey?