You should not have to go back and re-torque the valve cover fasteners. The bolts have a built in large diameter flange that push down the valve cover which then compresses the rubber valve cover gasket. The same flange on the bolt bottoms out on a metal sleeve in the holes of the valve cover. The metal sleeve is actually taller than the height of the valve cover. This ensures the exact proper compression of the rubber gasket, IE you can't over compress the gasket. As with any bolt, torque is created by the bolt stretch from bottoming out on the metal sleeve. In theory, they should not come un-torqued from the heat-cycling of the gasket and/or plastic valve cover.
As
@fmorelli said, having a good quality torque wrench is key with this. These are tiny fasteners with a low torque spec. 8.5 Nm (~45 in/lbs). I cheat a little and usually go 10 Nm. Its also important to note that cleanliness is key. It is a good idea to clean the bolts AND clean out the bolt holes in the head before re-installing the bolts. A touch of brake clean in the holes and blow them out with an air gun is what I do. On a street car I roll with it as is but on a race car I put loctite 243 (243 cures in the presence of oil) on the bolts.
People say these bolts should be replaced but BMW does not require it. They are steel and NOT a stretch bolt therefore no need to replace.
TIS says torque in a crosswise pattern going from inside to outside. (like a cylinder head to block) I don't have a Bentley so not sure what it says.