You would indeed still have to do tweaking.  Shrinkwrap does not give instant perfect results, especially in areas of folds or very detailed geometry on the underlyling mesh; it also tends to give odd-looking things around the armpit area that have to be corrected afterwards.  I generally find it most useful in combination with mirror (it never gives symmetric results otherwise) and subsurf (for fewer surface irregularities). 

It is not extrusion as I understand the term, but that might just reflect technical ignorance on my part.  In extrusion you move out from existing verts; shrinkwrap moves your new verts in where they fit right over the old ones (meaning those of the target mesh).