There's a reason the shoulders are not siped as deep as the centersection.
Excessive siping on the shoulders = chunking tires. I've hand siped two sets of mud tires for my winter recreational activities. I've only siped the inside lugs, never the outside lugs. Think about it. When you turn at highway speeds, where's the pressure go to on the tires? Get those tires hot and on dry pavement and you'll have chunks of rubber flying all over the place.
For the love of god, if you're going to hand sipe/groove tires, BUY A TIRE GROOVER. They are 60 bucks. If you do chose to use a dremel, make sure to take video.