Yes, definitely leave the backing paper on.
You may be interested to know that I had purchased some old Scott Stamp Mounts that were being discounted from a stamp dealer in a seemingly "odd" size of 40mm x 27mm, which are said to be the size for United Nations Commemoratives. They are just a fraction of a mm wider than a traditional stamp mount marketed for US Commemorative, but interestingly enough, they are exactly the perfect size for the US Self Adhesive Commemoratives as they have a slight bit more room to accommodate the backing paper. Of course, some collectors may prefer not to deviate from the traditional US Commemorative stamp mount size for the sake of consistency, but it does work.
