You want the lowest torque possible that prevents play in the headset. I just tightened mine by quarter-turns until there was zero play in the headset (pushing forwards with the brakes on), and make sure there's no creaking/binding.
Yep, last thing you want to do is to over-tighten. For the actual stem bolts I use a torque wrench set to 4Nm which should be plenty. Ignore the ratings on the stem if using a torque wrench. Also, maybe an obvious thing to some, but alternate between the bolts when tightening to get them as even as possible.