unlike Zbrush 2, Zb 3.1 and 3.5 don’t symetrize the center edgeloop of a model, (using smart Resym), when i use or not the posable symetry.
Not much comfort, but it’s a know issue. 
The fastest fix I use: Take your move tool and size down to 1 so you hit only a point a time. Go to level 1 and correct centerline by hand with the move tool. Go to level 2 repeat. Continue until your centerlines are clean. (Sometimes using the smooth brush over the area if the level above is cleanly centered will fix this.)
(Not my most favorite workaround, but life has become too short for me to fight with it anymore. 
)
-K