You get good quality mains only when they are fitted!!!. Else the mains supply will be loaded with spurious emissions.RFI filter, both ways - along with all the associated capacitors. In countries with good quality mains they often aren't fitted, or at least some of them aren't, with wire links replacing any series components passing current. It's common to see PCB's with loads of parts not fitted on the mains inputs, often there's space for a second filter as well.
It depends partially which way you're wanting to stop interference, very often in the UK with our high quality mains very few of the filter components are fitted, whereas the exact same devices in Spain (I've repaired quite a few devices sent back from Spain) have all the filter components fitted, and still commonly get blown up by mains issues.You get good quality mains only when they are fitted!!!. Else the mains supply will be loaded with spurious emissions.