If you want to mirror your geometry, and plan to make changes to it later (for example, if it's a mirrored circle right now, but you might change it later to a mirrored semicircle with a rectangular cutout in the middle), it's best to just make the sketch contain one half of the mirror, then after you extrude/revolve the shape, mirror the feature itself. That way you don’t have to make changes to duplicate copies of your sketch geometry.
It is also good practice to array your features at the part level, rather than at sketch level The only reason the sketch-level array has an "edit" option is so you can change the number of instances, the space between instances, or to suppress specific instances of your geometry. None of these options would even be applicable in the case of a mirror.
When you use the mirror command in a sketch, it is literally just making a copy of the sketch geometry and automatically applying symmetric constraints.
-Autodesk Inventor Professional 2012
