I have always been told to ignore and not use Mirror Geometry. Every person who has taught me Maya has stated:

Create 1 half
Duplicate it
Scale in X to -1
Combine and Merge Verts

The only problem with this is the time it takes to merge all the verts, its time consuming, but better off in the long run. If you Mirror you run the risk of spending the day removing and cleaning the gemometry around the seem to get your geo correct. that takes Longer.

Always use Combine mehod.