Mine is running pretty bad these days and I am thinking it is because of all of the oil 3 of the spark plugs are sitting in due to the bad valve cover gasket. For gaskets of this style I know that typically no additional sealant is used. However since these seem to all leak, would it be a good idea to add some kind of sealant when installing a new gasket since it is kind of a pain getting the intake off?

 

A leaking valve cover won't make it run bad. Even if there is oil sitting around the plug it won't make it run bad. You have other issues.
So to the valve cover leak I personally always add a thin layer to the the valve cover and let it get nice and tacky and then place the gasket on. Turn it over and place a weight on top to let it dry for about 30 min to an hour. (Time to drink a beer or two or three) Then install.

 

I've heard some people say that the oil being around the plug can cause a weak spark. I know oil isn't a good conductor, but it isn't a good insulator either so it sounded reasonable. I plan on doing a cylinder leak down test and check the fuel pressure before I replace the valve cover gasket.

So you put some sort of sealant on the valve cover, then put the rubber gasket on top and let it dry a bit. But then you don't add any sealant between the gasket and the head?

 

Correct ^^^^
That is how I do it. Others may say different. But I do this with valve cover, oil pan, and trans pan gaskets.