In the UK, where I live, none of this is necessary, so I've just been researching this subject. It seems that what you are doing, all parts of the procedure, are essential if you want your roses to survive undamaged through such hard winter weather. Primarily, mounding up is to protect the graft union, because if that gets damaged, the graft will fail; the loose litter protection added on top will help in protecting the buried stems from extra damage. Even if air temperatures are only -5 degrees, a wind chill factor of -20 degrees would actually dessicate, whiten and damage stems which are not adequately protected - plant stems can be just as vulnerable to wind chill and prone to damage in severe temperatures as any living thing would be were it not insulated in some fashion.
In fact, the method you've been using seems to be somewhat easier than the primary, trenching method recommended by the Minnesota Rose Society, and as it works (and follows the old and wise advice 'if it ain't broke, don't fix it') I wouldn't be changing any part of it if you want to keep your roses healthy. Their information is here: Winterizing Roses (Minnesota Rose Society). Similar advice is given by the University of Illinois, describing the same methods.
UPDATE: Further to your comments, I'd add that what you're doing when you protect your roses in this way is necessary because of where you live. The grafted part of the rose plant isn't something that will survive because of your cold winters, so if you want to grow plants like varieties of roses which would not naturally thrive on their own year round in your part of the world, then taking the steps you describe to protect them is, I'm afraid, unavoidable.
