I guess I've never seen any either. I tighten them until I can see the gasket just starting to squeeze. Over tightening will crush the gasket and cause leaks with steel covers like the rear one. If you have the cast iron front cover, then gasket crush isn't too much of an issue. The valve cover can be over tightened as well. The steel one will distort, the cast one will crack.

I know there are standard torque ratings for common bolts. Try this link out that I found through Google:
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