Just a little note for the peeps who do not know this yet!
If you plane to mirror an assembly, do NOT use planes for mating, the mates will NOT carry over into the mirrored Part.
I found this out, after my work partner had made an assembly, using this method. I wanted to change a distance mate, (this was between a plane and a surface), this worked on the original part, but the mirrored part stay the same!!
Every little helps! :-)