I wouldnt worry about cracking the cover itself, that seems like it would be hard to do unless you went a good bit over the torque specification. The valve cover does have gaskets built-in for the spark plugs though, and if any of those are leaking you must replace the whole cover. You can tell if they are leaking by oil getting into the spark plug tube.
Last year at some point another member asked about valve covers and I remember a few links in that thread for getting a set of replacement covers and gaskets for around $300 or so. You should look for that if you need to replace the covers at all because then you get everything for the price they want to charge you just for 1 cover.