Can I use 6-2 NM W/G when running a new 50A branch circuit to my cooktop?
I see no reason to include an insulated neutral. (40-years in the business.)  One thing you may or may not be aware of when making the splices in the J-box behind the new range, use an over size box. And #2 use something better than a large wire nut to make the hot lead connections.  Polaris makes a good splice connector. That kind of current can cause real problems with poor connections.  Wire nuts were never designed to handle those kind of loads.