MtBaker,
I've been looking for the mirror capability in V5 for several years, and I've come to the conclusion that it's best to just model each part separately.
I've been working with several plastic molded housings, and aluminum castings. The left and right handed parts are very similiar, but both contain minor differences. (ie: left housing has screw holes, and the right housing has screw bosses).
One alternative; is to add the symmetry at the bottom of the tree and just work with the reverse image of the part. I found this very difficult, especially when working with core parts that we boolean removed from the cavity shape.
Another alternative is to create a CATPart of the features that are the same on both parts. Then cut&paste with link the part into a left part and right part with one of the parts having the symmetry. Then add the unique features to the left and right parts.
Good luck! ....Jack