Back to the NAT...
if random tracking and crossing the NAT OTS enroute,its rare to get your optimum level as the OTS has priority. This is normally allowed for in the fuel planning.
If you have to transit the Santa Maria OCA (eg) UK to Caribbean, its even rarer to get a step climb unless its the controller's birthday when I believe they might let you up before reaching the NY OCA at 40 west!