The valve cover does not use a gasket, it is a seal. And because it is a square section rather than an O-ring, it really will not deform with extended use. So in my opinion, it is not necessary to replace it. The same valve cover seal has been used on C-14s since 2008 and there have not been any updates.
Kawasaki specifies a small amount of sealant be used in the radii of the seal where the camshaft bearings were bored in the head. I actually used a small bead of sealant all around the seal, between the valve cover and the seal itself, and when I install the cover I do not fully tighten the cover bolts- I leave them slightly loose (maybe 1/32 of an inch or so). Then after a few hours I tighten the valve cover bolts; this allows the RTV sealant to mostly cure and tends to force the seal to actually seal. C-14s have a bit of a reputation for some oil weepage in the front / left corner of the valve cover and so far the method outlined has worked for me. By the way, I use whatever brand of high temperature RTV sealant I happen to have on hand, and whatever color I happen to have.
The O-ring on the right side crankcase cover can be re-used also but it will tend to flat- spot over time. It is relatively easy to replace though so there is not much risk there even if you have to replace it later. I also use a dab (technical term: it means 'not too much') of RTV sealant where the O-ring crosses the rubber block that feeds the wires to the outside of the cover.
Brian
Just checked my valves at 25k on my '10 and they are all within spec.
Are people replacing the cover gasket and the o-ring on the side cover?
I have re-used gaskets like this in the past if they aren't damaged.
That said I don't want to dis-assemble the whole bike again if the cover
gasket leaks.