Hey Zeke,
I don't know what bike/engine you have so can't say whether missing washer is an issue. On the 750 engines I have done there were no washers on the valve cover screws. I can say that any bolt that starts to strip on reassembly WILL leak. Same thing happened to me, I had the engine 99% put together and one of the last two bolts(valve cover)stripped. I didn't want to go through the hassle of pulling everything apart, taking the head to be helicoiled, and springing for another head gasket, so I just let it be. Later in the life of this engine, one of the bolts in the spark plug recess began to pull loose and the head gasket blew. I am now of the mind that ANY stripped bolt should be taken as a sign of weakness/fatigue of the head and should be addressed before installing the engine. I'm not saying that your engine is going to blow, just relating what happened to me. And yes, higher speeds/ operating temps meant more oil weeping out. Hope this helps.