I used to have trouble with transmission pans. After I found we had an inch pound wrench I used the spec, typically 12 in lbs, found it took several rounds to get them all tight at once, no more leaks.
Same here, oil pans and valve covers too.
Type of pan/cover and gasket probably matters as well. If you're using a cork gasket with a stamped steel cover, then torque spec is crucial. I've been using cast aluminum covers/pans with anti-crush gaskets now for years and don't have any issues with hand tightening. Still using a stamped oil pan, but for that I bought Andy's pan reinforcements.
