Doing a SLOP is recommended a practise in MNPS regardless of whether it's a NAT-track or a random route, but it's main purpose is the track system though (so many aircrafts on the same route). Slop can be 0-1-2nm to the RIGHT only, and ATC does not have to be notified about it.
Link to the latest version of the MNPS manual 2013
http://www.icao.int/EURNAT/EUR%20and...0%20bkmrks.pdf