Thing I'm doing for devel (cross from linux, but sometimes running lazarus qt under win): Normal mingw installation of qt into some folder, copy all needed dll-s into eg C:\Qt4.XX and add it to the path, and then deinstall Qt installation.
For deployment: all needed dll's go inside application dir, so I don't touch C:\Windows\System at any time.Updating Qt version is easy, just push update into appdir, instead of updating each client machine.
Besides that from Qt >= 4.8.6 you must rebuild qt4pas.dll because it wont' work with precompiled dll from official bindings page since they changed mingw version for >= 4.8.6 build and some symbols cannot be found in qt4pas.dll