The reason I put rubber gaskets on my '81 R65, in 1992, is that they developed leaks there .
I had the cylinder heads overhauled in '98, severe valve recession, by San Jose BMW .
Got a call from the shop that I had performing the work on the heads, machinist said the top of the head, where the valve cover mates to, was warped and needed to be machined to true it up .
Machinist told me that it was a common problem on the airhead BMW's .
Take that for what it's worth .
The one thing you doan't want to do, is to overtighten the nut at the center of the valve cover, it's not uncommon to pull the stud out of the cylinder head if it's over tightened .