Depends on what kind of valve covers you have. With the carb I ran cheapie chrome stamped steel valve covers with the cheapie (is there a theme here?) cork valve covers. Put gasket sealer on the valve cover side of the gasket and could, and did, reuse them several times. I think one key to not having them leak is to straighten the edges of the valve cover, especially at the bolt holes. (see pic) I also used some valve cover washers I found at one of the box auto parts store. The were listed as being for a Chebby, but they worked great on my valve covers. The other thing is to not overtighten them. I torqued them to "one little finger on a 1/4" ratchet".
I now run the stock Ford 5.0 cast aluminum covers for my EFI. I use the FMS silicone valve cover gaskets, and... no leaks.
I never researched the torque requirements for the silicone gaskets, but my thought would be they require too much pressure to seal correctly with stamped steel valve covers.