Absolutely do not torque the cover bolts - just snug them and then a light finger pressure "nudge" on the end of a 8" handle should be quite sufficient. That's about the best I can describe the "feel" that I use. Any more and the gasket (and steel valve cover) will start to distort. With my mechanical lifter engine in the Boss I have my covers off fairly often and re-use the gaskets many times without problems.
Check out the little thumbwheels on the valve cover holddowns on the race car composite covers....just a good twist of the fingers and they're as tight as they need to be...I use rubber gaskets because the valve covers need to come off occasionally for maintenance...
Good twist of the fingers on the wingnuts is all you need...nothing heroic...*G*
Use torque spreaders under the wing nuts. These are available in several chrome varieties, short, long, shaped, etc. Any speed shop has them or can get them. What they do is to spread the clamping force over a much wider area of the valve cover preventing deformation but permitting a higher torque to be applied...the wider the torque spreaders the better...problem is to find ones that fit well and look good.