the trick is to understand how voxels work (v next to layer name). It is a 3d grid with uniformly sized "cells". So to get the finest quality you need to crank up voxel density prior to voxelizing your mesh object.
When it voxel mode fill all air pocket and other things that can cause issues, then turn your model into mesh again (you should get "s" next to layer name) then decimate to reasonable value. There's no single "correct" value - all depends on model scale and what your machine (3d printer / mill etc) can handle. You should aim to get as few polygons as possible - but enough to retain all the features you need.
For me voxels are one of the main reasons I have 3D Coat in my arsenal. It allows me to fix files that would be a major pita otherwise.